段错误是什么原因？

老哥，这题牛客光是报错段错误，不清楚什么原因。找了一个小时了都没发现什么数组越界的情况。

题目地址：https://www.nowcoder.com/practice/6a296eb82cf844ca8539b57c23e6e9bf?tpId=13&tqId=11182&rp=2&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking

代码原理堆排序：

class Solution {
public:
    vector<int> GetLeastNumbers_Solution( vector<int> input, int k ) {
        int len = input.size();
        if (len < k)
            return {};
        vector<int> res( input.begin(), input.begin() + k );
        //构造一个大小为k的大根堆，每次踢掉最大的，遍历完所有，最后留下的就是最小的k个
        for (int i = k - 1; i >= 1; i-=2) {
            helper( res, ( i - 1 ) >> 1,k);
        }
        for (int i = k; i < len; ++i) {
            if (res[0] > input[i]) {
                res[0] = input[i];
                helper(res,0,k);
            }
        }
        sort( res.begin(), res.end() );
        return res;
    }

    void helper( vector<int> &res,int parent,int k) {
        int left = ( parent << 1 ) + 1;
        if (left >= k)
            return;
        int right = left+1;
        if (right >= k) {
            if (res[parent] < res[left]) {
                swap( res[parent], res[left] );
                helper( res, left, k );
            }
            return;
        }
        if (res[left] > res[right]) {
            if (res[parent] < res[left]) {
                swap( res[parent], res[left] );
                helper( res, left, k );
            }
        } else {
            if (res[parent] < res[right]) {
                swap( res[parent], res[right] );
                helper( res, right, k );
            }
        }
    }
};