LeetCode-in-All

42. Trapping Rain Water

Hard

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:

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

Constraints:

• n == height.length
• 1 <= n <= 2 * 104
• 0 <= height[i] <= 105

Solution

import 'dart:math';

class Solution {
  int trap(List<int> heigth) {
    int left = 0,
        right = heigth.length - 1,
        leftMax = 0,
        rightMax = 0,
        water = 0;

    while (left < right) {
      leftMax = max(leftMax, heigth[left]);
      rightMax = max(rightMax, heigth[right]);
      water += leftMax < rightMax
          ? leftMax - heigth[left++]
          : rightMax - heigth[right--];
    }

    return water;
  }
}

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