Top k Largest Numbers

Given an integer array, find the top k largest numbers in it.

Example

Given [3,10,1000,-99,4,100] and k = 3.
Return [1000, 100, 10].

思路：由于需要按从大到小的顺序，因此直接用PriorityQueue即可，用Partition的方法的话还需要排序。直接用PriorityQueue 写的代码量少。

class Solution {
    /*
     * @param nums an integer array
     * @param k an integer
     * @return the top k largest numbers in array
     */
    public int[] topk(int[] nums, int k) {
        if (nums == null || nums.length < k) {
            return null;
        }
        
        int[] result = new int[k];
        PriorityQueue<Integer> minHeap = new PriorityQueue<>();
        for (int i = 0; i < nums.length; i++) {
            if (minHeap.size() < k) {
                minHeap.add(nums[i]);
            } else {
                int temp = minHeap.poll();
                minHeap.add(Math.max(temp, nums[i]));
            }
        }
        
        for (int i = k - 1; i >= 0; i--) {
            result[i] = minHeap.poll();
        }
        
        return result;
    }
};