Completed Array-1

Problem1.java

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+// Time Complexity : O(n) + O(n) ~ O(n)
+// Space Complexity : O(1)
+// Did this code successfully run on Leetcode : Yes
+// Three line explanation of solution in plain english
+
+// Your code here along with comments explaining your approach
+/*
+Build a prefix product array where res[i] stores the product of all numbers to the left of i.
+Then traverse from right side and multiply each res[i] with the running product of elements to the right of i.
+This way each index gets: (product of all left elements) × (product of all right elements) — without using division.
+*/
+
+public class Problem1 {
+    public int[] productExceptSelf(int[] nums) {
+        if(nums.length == 0) {
+            return new int[]{};
+        }
+        int n = nums.length;
+        int[] res = new int[n];
+        int runningProduct = 1;
+        res[0] = 1;
+        for(int i = 1;i<n;i++) {
+            res[i] = runningProduct * nums[i-1];
+            runningProduct = res[i];
+        }
+
+        runningProduct = 1;
+        for(int i = n-2;i>=0;i--) {
+            res[i] = (runningProduct * nums[i+1]) * res[i];
+            runningProduct *= nums[i+1];
+        }
+        return res;
+    }
+}

Problem2.java

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+// Time Complexity : O(M*N)
+// Space Complexity : O(M*N)
+// Did this code successfully run on Leetcode : Yes
+// Three line explanation of solution in plain english
+
+// Your code here along with comments explaining your approach
+/*
+Traverse the matrix diagonally, switching between upward and downward directions.
+When moving up, bounce off the top row or last column; when moving down, bounce off the bottom row or first column.
+Collect each cell in order while flipping the direction whenever a boundary is hit.
+*/
+
+public class Problem2 {
+    public int[] findDiagonalOrder(int[][] mat) {
+        int m = mat.length;
+        int n = mat[0].length;
+
+        int[] res = new int[m*n];
+        int row = 0;
+        int col = 0;
+        boolean goingUp = true;
+
+        for(int i = 0;i<m*n;i++) {
+            res[i] = mat[row][col];
+            if(goingUp) {
+                if(col == n-1) {
+                    goingUp = false;
+                    row++;
+                } else if(row == 0) {
+                    col++;
+                    goingUp = false;
+                } else {
+                    row--;
+                    col++;
+                }
+            } else {
+                if(row == m-1) {
+                    col++;
+                    goingUp = true;
+                } else if(col == 0) {
+                    row++;
+                    goingUp = true;
+                } else {
+                    col--;
+                    row++;
+                }
+            }
+        }
+
+        return res;
+    }
+}

Problem3.java

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+// Time Complexity : O(m * n)
+// Space Complexity : O(M*N)
+// Did this code successfully run on Leetcode : Yes
+// Three line explanation of solution in plain english
+
+// Your code here along with comments explaining your approach
+/*
+You maintain four boundaries (top, bottom, left, right) and traverse the matrix layer by layer.
+In each loop, you move right → down → left → up, shrinking the boundaries after each direction.
+You repeat this until all boundaries cross, ensuring every element is added exactly once in spiral order.
+ */
+
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Problem3 {
+    public List<Integer> spiralOrder(int[][] matrix) {
+        List<Integer> res = new ArrayList<>();
+        int m = matrix.length;
+        int n = matrix[0].length;
+
+        int left = 0;
+        int top = 0;
+        int right = n-1;
+        int bottom = m - 1;
+
+        while(left <= right && top <= bottom) {
+            for(int i = left;i<=right;i++) {
+                res.add(matrix[top][i]);
+            }
+            top++;
+
+            for(int i = top;i<=bottom;i++) {
+                res.add(matrix[i][right]);
+            }
+
+            right--;
+
+            if(top <= bottom) {
+                for(int i = right;i>=left;i--) {
+                    res.add(matrix[bottom][i]);
+                }
+
+                bottom--;
+            }
+
+            if(left <= right) {
+                for(int i = bottom;i>=top;i--) {
+                    res.add(matrix[i][left]);
+                }
+
+                left++;
+            }
+        }
+        return res;
+    }
+}