215. Kth Largest Element in an A

利用快速排序分区方法，
深入理解
return findXth(nums+index+1, numsSize-index-1,x-index-1);
else
return findXth(nums, index, x);

int partition(int a[], int low, int high)
{
        int pivot = a[low]; //挖坑


        while(low < high){
                for(;(low < high)&&(a[high] >= pivot); high--);
                a[low] = a[high]; //找到第一个小于pivot的填坑
                for(;(low < high)&&(a[low] <= pivot); low++);
                a[high] = a[low];//找到第一个大于pivot的填坑
        }
       //low high相交的时候，把low填入pivot,通知返回low坐标，low代表在排序数组中的下坐标 ，第low+1个
        a[low] = pivot;

        return low;
}

int findXth(int* nums, int numsSize, int x) {
    int index;
    index = partition(nums, 0, numsSize-1);

    if(index == x)
        return nums[index];

    if(index < x)
        return findXth(nums+index+1, numsSize-index-1,x-index-1);
    else
        return findXth(nums, index, x);
}

int findKthLargest(int* nums, int numsSize, int k) {

        return findXth(nums, numsSize, numsSize-k);
}