Add Trapping Rainwater problem implementation (Two Pointer Approach)

pom.xml

@@ -155,4 +155,4 @@
             </plugin>
         </plugins>
     </build>
-</project>
+</project>

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

@@ -0,0 +1,48 @@
+package com.thealgorithms.stacks;
+/**
+ * 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)
+ *
+ * Reference: https://en.wikipedia.org/wiki/Trapping_rain_water
+ */
+public final class TrappingRainwater {
+
+    private TrappingRainwater() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    public static int trap(int[] height) {
+        int left = 0;
+        int right = height.length - 1;
+        int leftMax = 0;
+        int rightMax = 0;
+        int result = 0;
+
+        while (left < right) {
+            if (height[left] < height[right]) {
+                if (height[left] >= leftMax) {
+                    leftMax = height[left];
+                } else {
+                    result += leftMax - height[left];
+                }
+                left++;
+            } else {
+                if (height[right] >= rightMax) {
+                    rightMax = height[right];
+                } else {
+                    result += rightMax - height[right];
+                }
+                right--;
+            }
+        }
+        return result;
+    }
+}

src/test/java/com/thealgorithms/stacks/TrappingRainwaterTest.java

@@ -0,0 +1,38 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class TrappingRainwaterTest {
+
+    @Test
+    public void testExampleCase() {
+        int[] height = {4, 2, 0, 3, 2, 5};
+        assertEquals(9, TrappingRainwater.trap(height));
+    }
+
+    @Test
+    public void testNoTrapping() {
+        int[] height = {1, 2, 3, 4, 5};
+        assertEquals(0, TrappingRainwater.trap(height));
+    }
+
+    @Test
+    public void testFlatSurface() {
+        int[] height = {0, 0, 0, 0};
+        assertEquals(0, TrappingRainwater.trap(height));
+    }
+
+    @Test
+    public void testSymmetricPit() {
+        int[] height = {3, 0, 2, 0, 3};
+        assertEquals(7, TrappingRainwater.trap(height));
+    }
+
+    @Test
+    public void testSingleBar() {
+        int[] height = {5};
+        assertEquals(0, TrappingRainwater.trap(height));
+    }
+}