LeetCode 42. Trapping Rain Water

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

• 0 <= n <= 3 * 10^4

• 0 <= height[i] <= 10^5

Solution

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class Solution {
public:
    int trap(vector<int>& height) {
        int i = 0;
        int j = height.size() - 1;
        int maxleft = 0, maxright = 0;
        int res = 0;
        while(i < j) {
            if(height[i] <= height[j]) {
                if (height[i] > maxleft) maxleft = height[i];
                else res += (maxleft - height[i]);
                i++;
            } else {
                if (height[j] > maxright) maxright = height[j];
                else res += (maxright - height[j]);
                j--;
            }
        }
        return res;
    }
};