Merge pull request #600 from harshitpandey-26/patch-3

Create Trapping_Rain_Water.java

Author: Pradeepsingh61

Java/algorithms/arrays/Trapping_Rain_Water.java

@@ -0,0 +1,46 @@
+/*
+Trapping Rain Water
+Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.
+*/
+
+class Solution {
+    public int trap(int[] arr) {
+        int n = arr.length;
+
+        // Edge case: If there are less than 3 bars, no water can be trapped
+        if (n < 3) return 0;
+
+        // ----------------------------
+        // Step 1: Create left max array
+        // ----------------------------
+        // left[i] stores the maximum height of bar from index 0 to i.
+        int[] left = new int[n];
+        left[0] = arr[0];
+        for (int i = 1; i < n; i++) {
+            left[i] = Math.max(left[i - 1], arr[i]);
+        }
+
+        // -----------------------------
+        // Step 2: Create right max array
+        // -----------------------------
+        // right[i] stores the maximum height of bar from index i to n-1.
+        int[] right = new int[n];
+        right[n - 1] = arr[n - 1];
+        for (int i = n - 2; i >= 0; i--) {
+            right[i] = Math.max(right[i + 1], arr[i]);
+        }
+
+        // -------------------------------------
+        // Step 3: Calculate trapped water at each index
+        // -------------------------------------
+        // The water trapped at each position depends on:
+        // min(max height on left, max height on right) - current height
+        int totalWater = 0;
+        for (int i = 0; i < n; i++) {
+            totalWater += Math.min(left[i], right[i]) - arr[i];
+        }
+
+        // Step 4: Return the total water trapped
+        return totalWater;
+    }
+}