diff --git a/pkg/trapping_rain_water/main.go b/pkg/trapping_rain_water/main.go
new file mode 100644
index 0000000..ba6a18c
--- /dev/null
+++ b/pkg/trapping_rain_water/main.go
@@ -0,0 +1,31 @@
+package trapping_rain_water
+
+func Trap(heights []int) int {
+	// Calculate area taken up by the boundary.
+	boundary_area := 0
+	for _, height := range heights {
+		boundary_area += height
+	}
+
+	// Calculate the total area taken up by the boundary and the water.
+	total_area := 0
+	left := 0
+	left_height := 0
+	right := len(heights) - 1
+	right_height := 0
+
+	for left <= right {
+		left_height = max(left_height, heights[left])
+		right_height = max(right_height, heights[right])
+
+		if left_height < right_height {
+			total_area += left_height
+			left++
+		} else {
+			total_area += right_height
+			right--
+		}
+	}
+
+	return total_area - boundary_area
+}
diff --git a/pkg/trapping_rain_water/main_test.go b/pkg/trapping_rain_water/main_test.go
new file mode 100644
index 0000000..3e1d7f2
--- /dev/null
+++ b/pkg/trapping_rain_water/main_test.go
@@ -0,0 +1,31 @@
+package trapping_rain_water
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTrap_Example1(t *testing.T) {
+	height := []int{0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1}
+	expected := 6
+	result := Trap(height)
+
+	assert.Equal(t, expected, result)
+}
+
+func TestTrap_Example2(t *testing.T) {
+	height := []int{4, 2, 0, 3, 2, 5}
+	expected := 9
+	result := Trap(height)
+
+	assert.Equal(t, expected, result)
+}
+
+func TestTrap_Example3(t *testing.T) {
+	height := []int{0}
+	expected := 0
+	result := Trap(height)
+
+	assert.Equal(t, expected, result)
+}