solve 42.trapping-rain-water using two pointers

Author: andavid

vscode/42.trapping-rain-water.1.java

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+/*
+ * @lc app=leetcode id=42 lang=java
+ *
+ * [42] Trapping Rain Water
+ *
+ * https://leetcode.com/problems/trapping-rain-water/description/
+ *
+ * algorithms
+ * Hard (44.33%)
+ * Likes:    4310
+ * Dislikes: 80
+ * Total Accepted:    339.3K
+ * Total Submissions: 761.1K
+ * Testcase Example:  '[0,1,0,2,1,0,1,3,2,1,2,1]'
+ *
+ * Given n non-negative integers representing an elevation map where the width
+ * of each bar is 1, compute how much water it is able to trap after raining.
+ *
+ *
+ * The above elevation map 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. Thanks
+ * Marcos for contributing this image!
+ *
+ * Example:
+ *
+ *
+ * Input: [0,1,0,2,1,0,1,3,2,1,2,1]
+ * Output: 6
+ *
+ */
+class Solution {
+  public int trap(int[] height) {
+    int n = height.length;
+    if (n <= 1) {
+      return 0;
+    }
+
+    int left = 0;
+    int right = n - 1;
+    int leftMax = height[left];
+    int rightMax = height[right];
+
+    int res = 0;
+    while (left <= right) {
+      leftMax = Math.max(leftMax, height[left]);
+      rightMax = Math.max(rightMax, height[right]);
+      if (leftMax < rightMax) {
+        res += leftMax - height[left];
+        left++;
+      } else {
+        res += rightMax - height[right];
+        right--;
+      }
+    }
+
+    return res;
+  }
+}
+