刷题739. Daily Temperatures

一、题目说明

题目739. Daily Temperatures，返回一列数，改天要等多少天才能升温。难度是Medium！

二、我的解答

这个题目，用dp解决非常方便：

class Solution{
	public:
		vector<int> dailyTemperatures(vector<int>& T){
			int len = T.size();
			vector<int> dp(len,0);
			dp[len-1] = 0;
			for(int i=len-2;i>=0;i--){
				if(T[i]<T[i+1]){
					dp[i] = 1;
				}else if(T[i]==T[i+1]){
					if(dp[i+1]==0){
						dp[i] = 0;
					} else{
						dp[i] = dp[i+1] + 1;
					}
				}else{
					for(int j=i+2;j<len;j++){
						if(T[i]<T[j]){
							dp[i] = j-i;
							break;
						}
					}
				}
			}
			return dp; 
		}
};

性能如下：

Runtime: 664 ms, faster than 5.08% of C++ online submissions for Daily Temperatures.
Memory Usage: 14.7 MB, less than 100.00% of C++ online submissions for Daily Temperatures.

三、优化措施

用一个单调递增栈，可以减少比较的次数：

class Solution{
	public:
		//promte by stack
		vector<int> dailyTemperatures(vector<int>& T){
			int len = T.size();
			vector<int> res(len,0);
			stack<int> st;
			for(int i=0;i<len;i++){
				while(! st.empty() && T[st.top()]<T[i]){
					res[st.top()] = i - st.top();
					st.pop();
				}
				st.push(i);
			}
			return res; 
		}
};

性能如下：

Runtime: 188 ms, faster than 90.99% of C++ online submissions for Daily Temperatures.
Memory Usage: 16.1 MB, less than 68.00% of C++ online submissions for Daily Temperatures.