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.

最简单的方法就是排序，倒数len-k就是结果。时间是O(nlog(n))。

public int findKthLargest(int[] nums, int k) {
    Arrays.sort(nums);
    return nums[nums.length-k];
}

还有类似与快速排序的方法。平均时间是O(n), 最坏的情况是O(n^2).

public int findKthLargest(int[] nums, int k) {
	if(nums==null||k<1)
		return 0;
	return getKth(nums.length-k,nums,0,nums.length-1);
}
private int getKth(int k,int[] nums,int start,int end){
	int result=nums[end];
	int left=start,right=end;
	while(left<right){
		while(nums[left]<result&&left<right)
			left++;
		while(nums[right]>=result&&left<right)
			right--;
		swap(nums,left,right);
	}
	swap(nums,left,end);
	if(k==left)
		return result;
	else if(k<left)
		return getKth(k,nums,start,left-1);
	else
		return getKth(k,nums,left+1,end);
}
private void swap(int[] nums,int a,int b){
	int tmp=nums[a];
	nums[a]=nums[b];
	nums[b]=tmp;
}