diff --git a/src/main/java/com/thealgorithms/searches/TrappingRainWater.java b/src/main/java/com/thealgorithms/searches/TrappingRainWater.java
new file mode 100644
index 000000000000..48d9391dab97
--- /dev/null
+++ b/src/main/java/com/thealgorithms/searches/TrappingRainWater.java
@@ -0,0 +1,53 @@
+package com.thealgorithms.searches;
+
+/**
+ * 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.
+ *
+ * Approach: Two-pointer optimized O(n) time and O(1) extra space.
+ */
+public final class TrappingRainWater {
+
+    private TrappingRainWater() {}
+
+    /**
+     * Compute trapped rain water amount for the given heights array.
+     *
+     * @param height array of non-negative integers
+     * @return total units of trapped water
+     * @throws IllegalArgumentException if height is null
+     */
+    public static int trap(final int[] height) {
+        if (height == null) {
+            throw new IllegalArgumentException("height array must not be null");
+        }
+
+        int left = 0;
+        int right = height.length - 1;
+        int leftMax = 0;
+        int rightMax = 0;
+        int trapped = 0;
+
+        while (left <= right) {
+            if (height[left] <= height[right]) {
+                if (height[left] >= leftMax) {
+                    leftMax = height[left];
+                } else {
+                    trapped += leftMax - height[left];
+                }
+                left++;
+            } else {
+                if (height[right] >= rightMax) {
+                    rightMax = height[right];
+                } else {
+                    trapped += rightMax - height[right];
+                }
+                right--;
+            }
+        }
+
+        return trapped;
+    }
+}
diff --git a/src/main/java/com/thealgorithms/strings/Upper.java b/src/main/java/com/thealgorithms/strings/Upper.java
index 5e248cb6ee39..14e6a4909ca8 100644
--- a/src/main/java/com/thealgorithms/strings/Upper.java
+++ b/src/main/java/com/thealgorithms/strings/Upper.java
@@ -19,21 +19,21 @@ public static void main(String[] args) {
      *
      * @param s the string to convert
      * @return the {@code String}, converted to uppercase.
+     * @throws IllegalArgumentException if {@code s} is null
      */
     public static String toUpperCase(String s) {
         if (s == null) {
-            throw new IllegalArgumentException("Input string connot be null");
+            throw new IllegalArgumentException("Input string cannot be null");
         }
         if (s.isEmpty()) {
             return s;
         }
-        StringBuilder result = new StringBuilder(s);
-        for (int i = 0; i < result.length(); ++i) {
-            char currentChar = result.charAt(i);
-            if (Character.isLetter(currentChar) && Character.isLowerCase(currentChar)) {
-                result.setCharAt(i, Character.toUpperCase(currentChar));
+        char[] chars = s.toCharArray();
+        for (int i = 0; i < chars.length; i++) {
+            if (Character.isLowerCase(chars[i])) {
+                chars[i] = Character.toUpperCase(chars[i]);
             }
         }
-        return result.toString();
+        return new String(chars);
     }
 }
diff --git a/src/test/java/com/thealgorithms/searches/TrappingRainWaterTest.java b/src/test/java/com/thealgorithms/searches/TrappingRainWaterTest.java
new file mode 100644
index 000000000000..6891b3499da7
--- /dev/null
+++ b/src/test/java/com/thealgorithms/searches/TrappingRainWaterTest.java
@@ -0,0 +1,38 @@
+package com.thealgorithms.searches;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+public class TrappingRainWaterTest {
+
+    @Test
+    public void testExample() {
+        int[] height = {4, 2, 0, 3, 2, 5};
+        assertEquals(9, TrappingRainWater.trap(height));
+    }
+
+    @Test
+    public void testEmpty() {
+        int[] height = {};
+        assertEquals(0, TrappingRainWater.trap(height));
+    }
+
+    @Test
+    public void testNoTrapping() {
+        int[] height = {0, 1, 2, 3, 4};
+        assertEquals(0, TrappingRainWater.trap(height));
+    }
+
+    @Test
+    public void testFlat() {
+        int[] height = {3, 3, 3, 3};
+        assertEquals(0, TrappingRainWater.trap(height));
+    }
+
+    @Test
+    public void testNull() {
+        assertThrows(IllegalArgumentException.class, () -> TrappingRainWater.trap(null));
+    }
+}