Add Trapping Rainwater problem implementation (Two Pointer Approach)

Author: Arzoo1701

src/main/java/com/thealgorithms/searches/TrappingRainwater.java

@@ -0,0 +1,58 @@
+package com.thealgorithms.searches;
+
+/**
+ * Trapping Rainwater Problem
+ * Given an array of non-negative integers representing the height of bars,
+ * compute how much water it can trap after raining.
+ * 
+ * Example:
+ * Input: [4,2,0,3,2,5]
+ * Output: 9
+ * 
+ * Time Complexity: O(n)
+ * Space Complexity: O(1)
+ */
+public class TrappingRainwater {
+
+    /**
+     * Calculates the total trapped rainwater.
+     *
+     * @param height an array representing elevation map
+     * @return total units of water trapped
+     */
+    public static int trap(int[] height) {
+        if (height == null || height.length == 0) {
+            return 0;
+        }
+
+        int left = 0, right = height.length - 1;
+        int leftMax = 0, rightMax = 0;
+        int trappedWater = 0;
+
+        while (left < right) {
+            if (height[left] < height[right]) {
+                if (height[left] >= leftMax) {
+                    leftMax = height[left];
+                } else {
+                    trappedWater += leftMax - height[left];
+                }
+                left++;
+            } else {
+                if (height[right] >= rightMax) {
+                    rightMax = height[right];
+                } else {
+                    trappedWater += rightMax - height[right];
+                }
+                right--;
+            }
+        }
+
+        return trappedWater;
+    }
+
+    // Example test
+    public static void main(String[] args) {
+        int[] height = {4, 2, 0, 3, 2, 5};
+        System.out.println("Total trapped water: " + trap(height)); // Output: 9
+    }
+}