Added trapping rainwater code

Leetcode/TrappingRainwater.java

@@ -0,0 +1,36 @@
+//https://leetcode.com/problems/trapping-rain-water/
+
+class Solution {
+    //T(n): O(n)
+    //S(n): O(n)
+    public int trap(int[] height) {
+        int n = height.length;
+        int[] left = new int[n];
+        int[] right = new int[n];
+
+        left[0] = height[0];
+        for (int i = 1; i < n; i++) {
+            if (height[i] > left[i - 1]) {
+                left[i] = height[i];
+            } else {
+                left[i] = left[i - 1];
+            }
+        }
+
+        right[n - 1] = height[n - 1];
+        for (int i = n - 2; i >= 0; i--) {
+            if (height[i] > right[i + 1]) {
+                right[i] = height[i];
+            } else {
+                right[i] = right[i + 1];
+            }
+        }
+
+        int sum = 0;
+        for (int i = 0; i < n; i++) {
+            height[i] = Math.min(left[i], right[i]) - height[i];
+            sum += height[i];
+        }
+        return sum;
+    }
+}