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Leetcode042-trappingRainWater

Posted on Edited on In Views: Valine:
Solution Report of LeetCode Acceptted

Description

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

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Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

Solution

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class Solution {
    public int trap(int[] height) {
        int res = 0;
        if (height == null || height.length == 0) return res;
        
        int i = 0, j = height.length-1;
        int leftmax = 0, rightmax = 0;
        while(i <= j){
            leftmax = Math.max(leftmax, height[i]);
            rightmax = Math.max(rightmax, height[j]);
            if (leftmax < rightmax){// leftmax is smaller than rightmax, so the (leftmax-A[a]) water can be stored
                res += leftmax - height[i];
                i++;
            }
            else{
                res += rightmax - height[j];
                j--;
            }
        }
        return res;
    }
}