求数组第K大

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.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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class Solution{public:    int findKthLargest(vector<int>& nums, int k)    {        int ans=  selectk(nums,0,nums.size()-1,k);        return nums[ans];    }    int selectk(vector<int>& a, int low, int high, int k)    {        if(k <= 0)            return -1;        if(k > high - low + 1)            return -1;        int pivot = low + rand()%(high - low + 1);    //随即选择一个支点        swap(a[low], a[pivot]);        int m = low;        int count = 1;        //一趟遍历，把较大的数放到数组的左边        for(int i = low + 1; i <= high; ++i)        {            if(a[i] > a[low])            {                swap(a[++m], a[i]);                count++;              //比支点大的数的个数为count-1            }        }        swap(a[m], a[low]);           //将支点放在左、右两部分的分界处        if(count > k)        {            return selectk(a, low, m - 1, k);        }        else if( count < k)        {            return selectk(a, m + 1, high, k - count);        }        else        {            return m;        }    }};