Task Scheduler

题目
Given a char array representing tasks CPU need to do. It contains capital letters A to Z where different letters represent different tasks.Tasks could be done without original order. Each task could be done in one interval. For each interval, CPU could finish one task or just be idle.

However, there is a non-negative cooling interval n that means between two same tasks, there must be at least n intervals that CPU are doing different tasks or just be idle.

You need to return the least number of intervals the CPU will take to finish all the given tasks.

Example 1:

Input: tasks = ["A","A","A","B","B","B"], n = 2
Output: 8
Explanation: A -> B -> idle -> A -> B -> idle -> A -> B.

Note:
The number of tasks is in the range [1, 10000].
The integer n is in the range [0, 100].

答案

public class Solution {
    public int leastInterval(char[] tasks, int n) {
        int[] count = new int[26];
        Map<Integer, Integer> cooldown = new HashMap<>();
        Queue<Integer> pq = new PriorityQueue<>((a, b) -> b - a);

        // Count occurences of each task
        for(int i = 0; i < tasks.length; i++)
            count[tasks[i] - 'A']++;

        // Put them into pq with equal priority
        for(int i = 0; i < count.length; i++) {
            if(count[i] != 0) pq.offer(count[i]);
        }

        int curr_time = 0;
        while(!pq.isEmpty() || !cooldown.isEmpty()) {
            // Add expired task to queue
            if(cooldown.containsKey(curr_time - n - 1))
                pq.offer(cooldown.remove(curr_time - n - 1));

            if(!pq.isEmpty()) {
                // Find task with largest remaining time
                int remain = pq.poll();
                if(remain - 1 > 0) {
                    cooldown.put(curr_time, remain - 1);
                }

            }

            curr_time++;
        }
        return curr_time;
    }
}