LeetCode 621. Task Scheduler

class Solution {
public:
    int leastInterval(vector<char>& tasks, int n) {
        priority_queue<int> q; // frequency of remaining tasks
        unordered_map<char,int> count;
        for (char task:tasks) ++count[task];
        for (auto x:count) q.push(x.second);
        
        int res=0;
        while (!q.empty()){
            // assign to n+1 slots according to frequency
            int slots=n+1;
            vector<int> new_freq;
            while (slots){
                if (q.empty()){
                    if (new_freq.size()==0)
                        return res;
                    else{
                        res += slots; // we need insert idles
                        break;
                    }
                }else{
                    int cur=q.top(); q.pop();
                    if (cur-1>0) new_freq.push_back(cur-1);
                    --slots; ++res;
                }
            }
            for (auto x:new_freq) q.push(x);
        }
        return res;
    }
};