621. Task Scheduler.cpp

// Solution 1. priority_queue.
class Solution {
public:
    int leastInterval(vector<char>& tasks, int n) {
        unordered_map<char, int>m;
        for(auto x: tasks) m[x]++;
        priority_queue<pair<int, char>>pq;
        for(auto x: m) pq.push(make_pair(x.second, x.first));
        int sum = 0;
        while(!pq.empty()){
            int time = 0;
            vector<pair<int, char>>v;
            for(int i = 0; i < n + 1; i++)
                if(!pq.empty()){
                    v.push_back(pq.top());
                    pq.pop();
                    time++;
                }
            for(auto x: v) if(--x.first) pq.push(x);
            sum += !pq.empty() ? n + 1 : time;
        }
        return sum;
    }
};

// Solution 2. Math
class Solution {
public:
    int leastInterval(vector<char>& tasks, int n) {
        unordered_map<char,int>m;
        int count = 0;
        for(auto x: tasks){
            m[x]++;
            count = max(count, m[x]);
        }
        int ans = (count-1) * (n + 1);
        for(auto x: m) if(x.second == count) ans++;
        return max((int)tasks.size(), ans);
    }
};