快速排序和找到第k大的数字

void quick_sort(int* A, int x, int y)
{
    if(y - x <= 1) return;  // 2 numbers at least
    int base = A[x];
    int p = x + 1, q = y - 1;
    while(p < q){
        while(p < q && A[q] >= base) q--; // only > is wrong, which result that q and p won‘t be changed, so dead loop
        while(p < q && A[p] < base) p++;
        swap(A[p], A[q]);
    }
    if(A[p] < A[x]) swap(A[x], A[p]);
    quick_sort(A, x, p);
    quick_sort(A, p, y);
}

int find_kth_number(int* A, int x, int y, int k)
{
    if(y - x == 1) return A[x];
    if(y - x < 1) return -1;
    int base = A[x];
    int p = x + 1, q = y - 1;
    while(p < q){
        while(p < q && A[q] >= base) q--;
        while(p < q && A[p] < base) p++;
        swap(A[p], A[q]);
    }
    if(A[p] < A[x]) swap(A[x], A[p]);
    if(p - 1 == k){ //一开始用了p和k坐比较，殊不知其实是左闭右开区间，即整个数组分成了x...p-1  p...y-1。一句话，分清界限
        return A[x];
    }else if(p - 1 < k){
        return find_kth_number(A, p, y, k);
    }else{
        return find_kth_number(A, x, p, k);
    }
}