刷题42. Trapping Rain Water

一、题目说明

题目是42. Trapping Rain Water，翻译起来就是“接雨水”。给n个非负正数代表高度，每个正数宽度为1，让计算能多少雨水。题目难度是Hard

二、我的解法

这个题目是找“坑”，然后计算里面可以存的“雨”。总共提交了5次，前面几次都是边界错误。

代码如下：

#include<iostream>
#include<vector>

using namespace std;
class Solution{
	public:
		int trap(vector<int>& height){
			if(height.size()<1) return 0;
			int len = height.size();
			int sum = 0,area=0,h;
			bool lflag = false,rflag = false;

			int left=0,leftStart,right,rightEnd=len-1,mid;

			while(left<rightEnd){
				//从左边开始找第1个高度
				leftStart = left;
				while(leftStart<len-1 && height[leftStart]<=height[leftStart+1]){
					leftStart++;
				}
				left = leftStart;

				//从右边开始找第1个高度
				right = rightEnd;
				while(right>left && height[right]<=height[right-1]){
					right--;
				}
				rightEnd = right;

				if(height[rightEnd]<=height[left]){
					right = rightEnd;
					//降
					while(right>left && (height[right]<=height[rightEnd])){
						right--;
					}
					//升
					while(right>left && (height[right]<height[right-1])){
						right--;
					}

					h = height[right]<height[rightEnd] ? height[right]: height[rightEnd];
					area = 0;
					for(int t=right+1;t<rightEnd;t++){
						if(h>height[t]){
							area = area + (h-height[t]);
						}
					}

					sum += area;
					rightEnd = right;
				}else{
					leftStart = left;
					//降
					while(left<rightEnd && (height[left]<=height[leftStart])){
						left++;
					}
					//升
					while(left<rightEnd && (height[left]<height[left-1])){
						left++;
					}

					h = height[left]<height[leftStart] ? height[left]: height[leftStart];
					area = 0;
					for(int t=leftStart+1;t<left;t++){
						if(h>height[t]){
							area = area + (h-height[t]);
						}
					}

					sum += area;
					leftStart = left;
				}
			}

			return sum;
		}
};
int main(){
	Solution s;
	vector<int> r;
	r = {0,1,0,2,1,0,1,3,2,1,2,1};
    cout<<s.trap(r)<<":"<<(6==s.trap(r))<<"\n";

    r = {5,4,1,2};
    cout<<s.trap(r)<<":"<<(1==s.trap(r))<<"\n";

    r = {5,2,1,2,1,5};
    cout<<s.trap(r)<<":"<<(14==s.trap(r))<<"\n";

    r = {5,5,1,7,1,1,5,2,7,6};
    cout<<s.trap(r)<<":"<<(23==s.trap(r))<<"\n";

    r = {6,4,2,0,3,2,0,3,1,4,5,3,2,7,5,3,0,1,2,1,3,4,6,8,1,3};
    cout<<s.trap(r)<<":"<<(83==s.trap(r))<<"\n";
	return 0;
}

性能如下：

Runtime: 8 ms, faster than 61.40% of C++ online submissions for Trapping Rain Water.
Memory Usage: 9.1 MB, less than 91.14% of C++ online submissions for Trapping Rain Water.

三、优化措施

代码虽然正确，但看起来很难过！多番寻找，相对优雅的代码如下：

class Solution{
	public:
		//left、right
		int trap(vector<int>& height) {
	        int n = height.size();
	        int lhigh = 0, rhigh = n-1;
	        int diff = 0;

	        // scan from left to right
	        for(int i = lhigh; i<n; i++)
	        {
	            if (height[i] < height[lhigh]) continue;
	            for (int j = lhigh+1; j<i; j++) diff += height[lhigh]-height[j];
	            lhigh = i;
	        }

	        // scan from right to left
	        for (int i = rhigh; i>=lhigh; i--)
	        {
	            if (height[i] < height[rhigh]) continue;
	            for (int j = i+1; j<rhigh; j++) diff += height[rhigh]-height[j];
	            rhigh = i;
	        }

	        return diff;
	    }
};

性能虽然差点，但可读性好多了。

Runtime: 12 ms, faster than 17.25% of C++ online submissions for Trapping Rain Water.
Memory Usage: 9 MB, less than 94.94% of C++ online submissions for Trapping Rain Water.