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704. Binary Search
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‎Leetcode/0704.Binary-Search/README.md‎

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# [704. Binary Search](https://leetcode.com/problems/binary-search/)
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## 題目
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Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1.
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You must write an algorithm with O(log n) runtime complexity.
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Example 1:
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```
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Input: nums = [-1,0,3,5,9,12], target = 9
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Output: 4
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Explanation: 9 exists in nums and its index is 4
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```
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Example 2:
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```
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Input: nums = [-1,0,3,5,9,12], target = 2
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Output: -1
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Explanation: 2 does not exist in nums so return -1
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```
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Constraints:
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* 1 <= nums.length <= 104
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* -104 < nums[i], target < 104
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* All the integers in nums are unique.
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* nums is sorted in ascending order.
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## 題目大意
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給一個已排序過後的array, 找出target所在index
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若未找到回傳 -1
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## 解題思路
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## 提示：
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## 來源
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* https://leetcode.com/problems/binary-search/
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## 解答
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https://github.com/kimi0230/LeetcodeGolang/blob/master/Leetcode/0704.Binary-Search/main.go
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```go
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package binarysearch
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import "sort"
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func Search(nums []int, target int) int {
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lenght := len(nums)
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if lenght <= 0 {
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return -1
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}
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left, right := 0, lenght-1
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for left <= right {
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mid := (right-left)/2 + left
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if nums[mid] == target {
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return mid
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} else if nums[mid] < target {
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// 找右邊
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left = mid + 1
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} else if nums[mid] > target {
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// 找左邊
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right = mid - 1
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}
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}
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// 都沒找到
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return -1
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}
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func Search2(nums []int, target int) int {
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lenght := len(nums)
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if lenght <= 0 {
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return -1
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}
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left, right := 0, lenght-1
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for left <= right {
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// 除以2
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// mid := left + (right-left)>>1
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mid := int(uint(right+left) >> 1)
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if nums[mid] == target {
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return mid
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} else if nums[mid] < target {
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// 找右邊
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left = mid + 1
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} else if nums[mid] > target {
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// 找左邊
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right = mid - 1
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}
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}
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// 都沒找到
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return -1
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}
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// 內建sort
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func BinarySearch(nums []int, target int) int {
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length := len(nums)
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index := sort.Search(length, func(i int) bool {
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return nums[i] >= target
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})
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if index < length && nums[index] == target {
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return index
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} else {
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return -1
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}
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}
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// 使用遞迴
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func BinarySearchRecur(nums []int, target int) (index int) {
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return BinarySearchRecursively(nums, target, 0, len(nums)-1)
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}
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func BinarySearchRecursively(nums []int, target int, start int, end int) int {
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if end < start {
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return -1
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}
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middle := int(uint(end+start) >> 1)
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if nums[middle] == target {
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return middle
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} else if target > nums[middle] {
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return BinarySearchRecursively(nums, target, middle+1, end)
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} else {
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return BinarySearchRecursively(nums, target, start, middle-1)
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}
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}
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```

‎Leetcode/0704.Binary-Search/main.go‎

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package binarysearch
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import "sort"
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func Search(nums []int, target int) int {
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lenght := len(nums)
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if lenght <= 0 {
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return -1
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}
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left, right := 0, lenght-1
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for left <= right {
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mid := (right-left)/2 + left
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if nums[mid] == target {
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return mid
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} else if nums[mid] < target {
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// 找右邊
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left = mid + 1
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} else if nums[mid] > target {
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// 找左邊
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right = mid - 1
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}
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}
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// 都沒找到
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return -1
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}
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func Search2(nums []int, target int) int {
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lenght := len(nums)
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if lenght <= 0 {
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return -1
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}
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left, right := 0, lenght-1
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for left <= right {
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// 除以2
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// mid := left + (right-left)>>1
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mid := int(uint(right+left) >> 1)
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if nums[mid] == target {
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return mid
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} else if nums[mid] < target {
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// 找右邊
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left = mid + 1
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} else if nums[mid] > target {
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// 找左邊
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right = mid - 1
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}
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}
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// 都沒找到
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return -1
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}
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// 內建sort
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func BinarySearch(nums []int, target int) int {
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length := len(nums)
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index := sort.Search(length, func(i int) bool {
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return nums[i] >= target
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})
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if index < length && nums[index] == target {
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return index
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} else {
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return -1
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}
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}
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// 使用遞迴
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func BinarySearchRecur(nums []int, target int) (index int) {
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return BinarySearchRecursively(nums, target, 0, len(nums)-1)
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}
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func BinarySearchRecursively(nums []int, target int, start int, end int) int {
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if end < start {
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return -1
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}
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middle := int(uint(end+start) >> 1)
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if nums[middle] == target {
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return middle
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} else if target > nums[middle] {
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return BinarySearchRecursively(nums, target, middle+1, end)
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} else {
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return BinarySearchRecursively(nums, target, start, middle-1)
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}
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}

‎Leetcode/0704.Binary-Search/main_test.go‎

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package binarysearch
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import "testing"
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var tests = []struct {
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arg1 []int
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arg2 int
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want int
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}{
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{
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[]int{-1, 0, 3, 5, 9, 12},
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9,
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4,
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},
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{
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[]int{-1, 0, 3, 5, 9, 12},
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2,
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-1,
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},
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{
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[]int{-1, 0, 3, 5, 9, 12},
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13,
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-1,
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},
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}
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func TestSearch(t *testing.T) {
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for _, tt := range tests {
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if got := Search(tt.arg1, tt.arg2); got != tt.want {
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t.Errorf("got = %v, want = %v", got, tt.want)
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}
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}
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}
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func TestSearch2(t *testing.T) {
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for _, tt := range tests {
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if got := Search2(tt.arg1, tt.arg2); got != tt.want {
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t.Errorf("got = %v, want = %v", got, tt.want)
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}
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}
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}
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func TestBinarySearch(t *testing.T) {
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for _, tt := range tests {
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if got := BinarySearch(tt.arg1, tt.arg2); got != tt.want {
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t.Errorf("got = %v, want = %v", got, tt.want)
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}
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}
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}
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func TestBinarySearchRecur(t *testing.T) {
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for _, tt := range tests {
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if got := BinarySearchRecur(tt.arg1, tt.arg2); got != tt.want {
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t.Errorf("got = %v, want = %v", got, tt.want)
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}
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}
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}
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func BenchmarkSearch(b *testing.B) {
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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Search(tests[0].arg1, tests[0].arg2)
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}
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}
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func BenchmarkSearch2(b *testing.B) {
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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Search2(tests[0].arg1, tests[0].arg2)
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}
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}
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func BenchmarkBinarySearch(b *testing.B) {
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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BinarySearch(tests[0].arg1, tests[0].arg2)
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}
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}
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func BenchmarkBinarySearchRecur(b *testing.B) {
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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BinarySearchRecur(tests[0].arg1, tests[0].arg2)
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}
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}
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/*
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go test -benchmem -run=none LeetcodeGolang/Leetcode/0704.Binary-Search -bench=.
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goos: darwin
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goarch: amd64
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pkg: LeetcodeGolang/Leetcode/0704.Binary-Search
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cpu: Intel(R) Core(TM) i5-6400 CPU @ 2.70GHz
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BenchmarkSearch-4 365892498 3.483 ns/op 0 B/op 0 allocs/op
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BenchmarkSearch2-4 405742570 2.734 ns/op 0 B/op 0 allocs/op
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BenchmarkBinarySearch-4 81949826 13.15 ns/op 0 B/op 0 allocs/op
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BenchmarkBinarySearchRecur-4 253851086 4.588 ns/op 0 B/op 0 allocs/op
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*/

‎README.md‎

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- [Bit Manipulation](#bit-manipulation)
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- [Union Find](#union-find)
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- [Breadth First Search](#breadth-first-search)
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- [Binary Search](#binary-search)
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- [GeeksforGeeks Content](#geeksforgeeks-content)
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- [Codility Content](#codility-content)
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- [Reference](#reference)
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| 0310 | [Minimum Height Trees](https://leetcode.com/problems/minimum-height-trees/) | [Go](https://github.com/kimi0230/LeetcodeGolang/tree/master/Leetcode/0310.Minimum-Height-Trees) | Medium | | | Breadth First Search |
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| 0752 | [752. Open the Lock](https://leetcode.com/problems/open-the-lock/) | [Go](https://github.com/kimi0230/LeetcodeGolang/tree/master/Leetcode/0752.Open-the-Lock) | Medium | | | Breadth First Search |
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---
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#### Binary Search
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分析二分搜尋技巧: 不要出現 **else**, 而是把所有情況用 **else if** 寫清楚.
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計算 mid 時需要防止溢出
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| No. | Title | Solution | Difficulty | Time | Space | Topic |
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|-----|:-----:|:--------:|------------|------|-------|-------|
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| 0704 | [704. Binary Search](https://leetcode.com/problems/binary-search/) | [Go](https://github.com/kimi0230/LeetcodeGolang/tree/master/Leetcode/0704.Binary-Search) | Easy | 最差:O(long n)<br> 最佳O(1)剛好在中間 | 迭代: O(1) <br/> 遞迴O(log n) | Binary Search |
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---
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## [GeeksforGeeks](https://www.geeksforgeeks.org/) Content

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