获取数组的前K小的元素

//升序排列
template 
int partition(vector& v, int left, int right) {
    if (left >= right) return left;
    int i = left, j = right;
    Comparable pivot = v[left];
    while (i < j) {
        while (i < j && !(v[j] < pivot)) j--;
        v[i] = v[j];
        while (i < j && v[i] < pivot) i++;
        v[j] = v[i];
    }
    v[i] = pivot;
    return i;
}

template 
void getTopK(vector& v, int k) {
    int N = (int)v.size();
    if (N <= k) {
        return;
    }
    int left = 0;
    int right = N - 1;
    int mid = partition(v, left, right);
    //不断调整分治的位置，直到position = k-1
    //大了，往前调整
    //小了，往后调整
    while (mid != k - 1) {
        if (mid > k - 1) {
            right = mid - 1;
            mid = partition(v, left, right);
        } else {
            left = mid + 1;
            mid = partition(v, left, right);
        }
    }
    v.resize(k);
}