Add test cases for TrappingRainwater algorithm

Author: Arzoo1701

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

@@ -0,0 +1,37 @@
+package com.thealgorithms.searches;
+
+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));
+    }
+}