Create 0042_Trapping_Rain_Water.java

0042_Trapping_Rain_Water.java

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+// id: 42
+// Name: Trapping Rain Water
+// link: https://leetcode.com/problems/trapping-rain-water/description/
+// Difficulty: Hard
+
+class Solution {
+    public int trap(int[] height) {
+        int n = height.length;
+        int [] rightMaxes = new int[n]; // maximum height at right of index i
+        rightMaxes[n-1] = 0; // no element right
+
+        for (int i = n-2; i >= 0; i--) {
+            rightMaxes[i] = Math.max(height[i+1], rightMaxes[i+1]);
+        }
+
+
+        int result = 0;
+        int leftMax = height[0];
+        for (int i = 1; i < height.length-1; i++) {
+            // max right, max left
+            int current = Math.min(rightMaxes[i], leftMax) - height[i];
+
+            if (current > 0) result += current;
+
+            leftMax = Math.max(leftMax, height[i]);
+        }
+
+        return result;
+    }
+}