Comparator for min-heap in C++

问题

I am trying to make a min-heap¹ of longs in C++ using the STL make_heap, etc., but my comparator doesn't seem to be comparing properly. The following is my current comparator:

struct greater1{
    bool operator()(const long& a,const long& b) const{
        return a>b;
    }
};

However, when I do std::pop_heap(humble.begin(),humble.end(),g); where g is an instance of greater1 and humble is a heap who makes [9,15,15,25] when sort_heap is called, I get a 15 popped.

Is my comparator correct? what might be going wrong?

EDIT:
I realized that I am running sort_heap with no comparator, whereas when I run it this comparator, I get [15,15,9,25] from sort_heap. Now I am thinking my comparator is definitely not working, but unsure why.

¹The STL makes a max-heap by default, so I need a comparator.

回答1:

Perhaps you are missing something somewhere, the code below works as intended:

#include <vector>
#include <algorithm>
#include <iostream>

struct greater1{
  bool operator()(const long& a,const long& b) const{
    return a>b;
  }
};

int main() {
  std::vector<long> humble;
  humble.push_back(15);
  humble.push_back(15);
  humble.push_back(9);
  humble.push_back(25);

  std::make_heap(humble.begin(), humble.end(), greater1());
  while (humble.size()) {
    std::pop_heap(humble.begin(),humble.end(),greater1());
    long min = humble.back();
    humble.pop_back();  
    std::cout << min << std::endl;
  }

  return 0;
}

回答2:

just use greater<int>(). it is predefined in std.

回答3:

You want to call make_heap on the vector again, not sort_heap. make_heap will rearrange your entire vector into a min heap given the greater-than comparator whereas sort_heap sorts your element into ascending order and is no longer a heap at all!

#include <algorithm>
#include <iostream>
#include <vector>

struct greater1{
    bool operator()(const long& a,const long& b) const{
        return a>b;
    }
};

int main()
{
  unsigned int myints[] = {10,20,30,5,15};
  vector<unsigned int> v(myints, myints+5);

  //creates max heap
  std::make_heap(v.begin(). v.end()); // 30 20 10 5 15

  //converts to min heap
  std::make_heap(v.begin(). v.end(), greater1()); // 5 15 10 20 30

  unsigned int s =  v.size();

  //ALSO NEED TO PASS greater1() to pop()!!!
  for(unsigned int i = 0; i < s; i++)
    std::pop_heap(v.begin(). v.end(), greater1()); // popping order: 5 10 15 20 30

  return 0;
}

来源：https://stackoverflow.com/questions/14016921/comparator-for-min-heap-in-c

标签

c++

stl

heap

comparator

min-heap