C++ 堆排序 以及用堆排序解决topk问题

void siftdown(vector<int>& nums, int e, int begin, int end){
    int i = begin;
    int j = 2*begin + 1;
    while(j<end){
        if(j+1 < end && nums[j+1] > nums[j]){
            j++;
        }
        if(e > nums[j]){
            break;
        }
        nums[i] = nums[j];
        i = j;
        j = 2*i + 1;
    }
    nums[i] = e;
}

void heap_sort(vector<int>& nums){
    auto end = (int)nums.size();
    for(auto k = end/2; k>=0;k--){
        siftdown(nums, nums[k], k, end);
        auto n = nums;
        int i = 0;
    }
    for(int k = end - 1;k>0;k--){
        int e = nums[k];
        nums[k] = nums[0];
        siftdown(nums, e, 0, k);
    }
}

int heap_sort_topk(vector<int>& nums, int topk){
    auto len = (int)nums.size();
    if(topk > len){
        return 0;
    }
    for ( int k = (len/2); k > 0; k-- ){
        sift_down(nums, nums[k], k, len);
    }

    for( int k = len -1; k > 0; k--){
        if ( topk < 0){
            return nums[0];
        }
        sift_down(nums, nums[k], 0, k);
        topk--;
    }
}