Add solution of TrappingRainWater task

Author: andrei-punko

src/main/java/by/andd3dfx/common/TrappingRainWater.java

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+package by.andd3dfx.common;
+
+/**
+ * <pre>
+ * <a href="https://leetcode.com/problems/trapping-rain-water/description/">Task description</a>
+ *
+ * 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.
+ *
+ * Example 1:
+ * <img src="https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png" />
+ *
+ * Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
+ * Output: 6
+ * Explanation: The above elevation map (black section) is represented
+ * by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
+ *
+ * Example 2:
+ * Input: height = [4,2,0,3,2,5]
+ * Output: 9
+ * </pre>
+ */
+public class TrappingRainWater {
+
+    public static int trap(int[] height) {
+        if (height == null || height.length == 0) {
+            return 0;
+        }
+
+        int n = height.length;
+        int[] leftMax = new int[n];
+        int[] rightMax = new int[n];
+        int water = 0;
+
+        // Calculate left_max
+        leftMax[0] = height[0];
+        for (int i = 1; i < n; i++) {
+            leftMax[i] = Math.max(height[i], leftMax[i - 1]);
+        }
+
+        // Calculate right_max
+        rightMax[n - 1] = height[n - 1];
+        for (int i = n - 2; i >= 0; i--) {
+            rightMax[i] = Math.max(height[i], rightMax[i + 1]);
+        }
+
+        // Calculate trapped water
+        for (int i = 0; i < n; i++) {
+            water += Math.min(leftMax[i], rightMax[i]) - height[i];
+        }
+
+        return water;
+    }
+}

src/test/java/by/andd3dfx/common/TrappingRainWaterTest.java

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+package by.andd3dfx.common;
+
+import static by.andd3dfx.common.TrappingRainWater.trap;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class TrappingRainWaterTest {
+
+    @Test
+    public void testTrap() {
+        assertThat(trap(new int[]{1, 0, 1})).isEqualTo(1);
+        assertThat(trap(new int[]{1, 0, 2})).isEqualTo(1);
+        assertThat(trap(new int[]{3, 0, 2})).isEqualTo(2);
+        assertThat(trap(new int[]{1, 3, 1, 2, 0, 1})).isEqualTo(2);
+
+        assertThat(trap(new int[]{0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1})).isEqualTo(6);
+        assertThat(trap(new int[]{4, 2, 0, 3, 2, 5})).isEqualTo(9);
+    }
+}