heap priority queue

Given a letter (A to Z) array representing tasks. 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

tasks = ["A","A","A","B","B","B"], n = 2 -> 8

A-B-#-A-B-#-A-B

Solution

Use a max-heap to place most frequent elements in the first priority.

def leastInterval(tasks, n):
    n += 1
    ret = 0
    d = collections.Counter(tasks)
    heap = [-c for c in d.values()] # max-heap build trick
    heapq.heapify(heap)
    while heap:
        temp = []
        count = 0
        for _ in range(n):
            if heap:
                c = heapq.heappop(heap)
                count += 1
                if c < -1:
                    temp.append(c + 1)
        for item in temp:
            heapq.heappush(heap, item)
        ret += n if heap else count
    return ret