Add Trapping Rain Water algorithm in Java

Author: SuryanshuAgrawal

src/main/java/com/thealgorithms/dynamicprogramming/TrappingRainWater.java

@@ -0,0 +1,56 @@
+package com.thealgorithms.dynamicprogramming;
+
+public class TrappingRainWater {
+
+    /**
+     * Calculates total water trapped between the pillars.
+     *
+     * @param height array of non-negative integers representing pillar heights
+     * @return total units of trapped water
+     */
+    public static int trap(int[] height) {
+        int n = height.length;
+        if (n == 0) return 0;
+
+        int sum = 0;
+        int topmax = 0;
+        int temp2 = 0;
+
+        // Find the index of the tallest pillar
+        for (int i = 0; i < n; i++) {
+            if (height[i] > temp2) {
+                topmax = i;
+                temp2 = height[i];
+            }
+        }
+
+        int temp = height[0];
+
+        for (int i = 0; i < n; i++) {
+            if (i == 0) {
+                sum += Math.min(height[i], height[topmax]) - height[i];
+            } else if (i > 0 && i < topmax) {
+                if (temp < height[i]) {
+                    temp = height[i];
+                    sum += Math.min(height[i], height[topmax]) - height[i];
+                } else {
+                    sum += Math.min(temp, height[topmax]) - height[i];
+                }
+            } else if (i == topmax) {
+                // do nothing
+            } else if (i > topmax) {
+                int temp1 = height[i];
+                int j = i;
+                while (j < n - 1) {
+                    if (temp1 < height[j + 1]) {
+                        temp1 = height[j + 1];
+                    }
+                    j++;
+                }
+                sum += Math.min(height[topmax], temp1) - height[i];
+            }
+        }
+
+        return sum;
+    }
+}