剣指offerスライドウィンドウの最大値

class Solution {
public:
    vector<int> maxInWindows(const vector<int>& a, unsigned int k){
        vector<int> res;
        deque<int> s;
        for(unsigned int i = 0; i < a.size(); ++i){
            while(s.size() && a[s.back()] <= a[i]) s.pop_back();
            while(s.size() && i - s.front() + 1 > k) s.pop_front();
            s.push_back(i);
            if(k && i + 1 >= k) res.push_back(a[s.front()]);
        }
        return res;
    }
};