215. Kth Largest Element in an Array

Posted 2020-08-10 Machelsky

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note: 
You may assume k is always valid, 1 ≤ k ≤ array‘s length.

思路：用priorityqueue建maxheap然后读出第几个。

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        if(k>nums.length)
        {
            return 0;
        }
        PriorityQueue<Integer> res=new PriorityQueue<>((a,b)->(b-a));
        for(int i=0;i<nums.length;i++)
        {
            res.offer(nums[i]);
        }
        int m=0;
        int count=0;
        while(count<k)
        {
            m=res.poll();
            count++;
        }
        return m;
    }
}

Solution2:

discussion的一种解法特别巧妙。用java自带的minheap，size小于k就加数，大于k就减掉最小的，这样子就能保证queue最顶端的就是最k大的。最后退出循环返回的就是第k大的数。

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        if(k>nums.length)
        {
            return 0;
        }
        PriorityQueue<Integer> res=new PriorityQueue<>();
        for(int i=0;i<nums.length;i++)
        {

            res.offer(nums[i]);
            if(res.size()>k)
            {
                res.poll();
            }
        }
        return res.poll();
    }
}