739 - Daily Temperatures

#medium

Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the ith day to get a warmer temperature. If there is no future day for which this is possible, keep answer[i] == 0 instead.

Example 1:
Input: temperatures = [73,74,75,71,69,72,76,73]
Output: [1,1,4,2,1,1,0,0]

Example 2:
Input: temperatures = [30,40,50,60]
Output: [1,1,1,0]

Example 3:
Input: temperatures = [30,60,90]
Output: [1,1,0]

class Solution {
public:
    vector<int> dailyTemperatures(vector<int>& temperatures) {
        // 單調遞增的stack，stack裡存的是index
        vector<int> result( temperatures.size(), 0 );
        stack<int> st;
        for ( int i = 0; i < temperatures.size(); i++ ) {
            while ( !st.empty() && temperatures[i] > temperatures[st.top()] ) {
                result[st.top()] = i - st.top();
                st.pop();
            }

            st.push( i );
        }

        return result;
    }
};