什么是自定义比较器以及如何在 C++ 的排序函数中使用它？

Question

 vector<vector<int>> reconstructQueue(vector<vector<int>> &people) {
    auto cmp = [](const vector<int> &a, const vector<int> &b) {
        return a[0] > b[0] || (a[0] == b[0] && a[1] < b[1]);
    };
    sort(people.begin(), people.end(), cmp);

Hey guys can someone explain me the auto cmp... part and how to use it in a sort function any help will be highly appreciated :)

Answer 1

This part of the code is a lambda . In other words, a small function defined locally. You can make a search for this term for more info.

IN the case of the sort algorithm, you are expected to pass a function taking two arguments and returning true if the first argument is less than the second. To make it simple, you give an alternative comparison function to use instead of the standard operator< aka std::less. Any object having an operator() can be used.

Answer 2

The comparator can be implemented in multiple ways, for eg. a lambda (as in your example) or a callable struct/class. Just to give you an example for sorting say a vector of std::pair by the second element:

struct Comparator
{
    bool operator()(const std::pair<int,int>& lhs, const std::pair<int,int>& rhs)
    {
        return lhs.second < rhs.second;
    }
}
void foo(std::vector<std::pair<int,int> > &v)
{
    std::sort(v.begin(), v.end(), Comparator);
}

Alternatively, instead of using a callable struct, you can write foo using lambda as:

void foo(std::vector<std::pair<int,int> > &v)
{
    std::sort(v.begin(), v.end(), 
        [](const pair<int, int>& lhs, const pair<int, int>& rhs) 
           {
             return lhs.second < rhs.second; 
           } );
}

You can write more advanced comparisons by say capturing some other data and using that for comparisons. Try reading about lambdas .