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215. Kth Largest Element in an Array

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();    }}

 

215. Kth Largest Element in an Array