Given a list of daily temperatures T, return a list such that, for each day in the input, tells you how many days you would have to wait until a warmer temperature. If there is no future day for which this is possible, put 0 instead.

For example, given the list of temperatures T = [73, 74, 75, 71, 69, 72, 76, 73], your output should be [1, 1, 4, 2, 1, 1, 0, 0].

Note: The length of temperatures will be in the range [1, 30000]. Each temperature will be an integer in the range [30, 100].

Solution:

Iterate backwards and maintain a stack, pop until stack top is more than current element.

class Solution {
public:
    vector<int> dailyTemperatures(vector<int>& T) {
        stack<pair<int,int>> s;
        vector<int> ans(T.size());
        for(int i = T.size()-1; i>=0; i--){
            while(!s.empty() && s.top().first <= T[i]){
                s.pop();
            }
            if(s.empty()) ans[i] = 0;
            else ans[i]=s.top().second-i;
            s.push(make_pair(T[i],i));
        }
        return ans;
    }
};