42. Trapping Rain Water (JAVA)

佚名 6年前 (2019-05-05) 随笔 1362人围观抢沙发百度已收录

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.

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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

首先找到最高的bar，然后从左到这个最高的bar，依次计算面积；再从右到这个最高的bar，依次计算面积。时间复杂度n*2 = O(n)

class Solution {
    public int trap(int[] height) {
        int ret = 0;
        int curIndex = 0; //index of highest 
        int topIndex = 0;
        for(int i = 1; i < height.length; i++){
            if(height[i] > height[topIndex]) topIndex = i;
        }
        
        for(int i = 0; i < topIndex; i++){
            if(height[i] > height[curIndex]){
                curIndex = i;   
            }
            else{
                ret += (height[curIndex]-height[i]);
            }
        }
        
        curIndex = height.length-1;
        for(int i = height.length-2; i > topIndex; i--){
            if(height[i] > height[curIndex]){
                curIndex = i;   
            }
            else{
                ret += (height[curIndex]-height[i]);
            }
        }
        
        return ret;
    }
}