Monotonic queue

A 4th question of Biweekly LeetCode contest.

1944. Number of Visible People in a Queue

class Solution {
public:
vector<int> canSeePersonsCount(vector<int>& heights) {
const int n = heights.size();
vector<int> ret(n);
vector<int> q;
for (int i = n - 1; i >=0; --i){
while (!q.empty() && heights[i] > q.back()){
ret[i]++;
q.pop_back();
}
if (!q.empty())ret[i]++;
q.push_back(heights[i]);
}
return ret;
}
};