优先队列

优先队列

c++ STL 中 优先队列用法(默认为less，最大堆)，头文件为需要包括 :

#include <queue>
/*
*  std::priority_queue:
*  template <class T, class Container = vector<T>,
*  class Compare = less<typename Container::value_type> > class *  priority_queue;
*/

例子：

int a[7] = {1,4,5,2,8,3,0};
priority_queue <int, vector<int>, less<int> > p;
priority_queue <int, vector<int>, greater<int> >q;
for(int i = 0; i < 5; i++)
    {
        p.push(a[i]);
        q.push(a[i]);
    }
//output p:8 5 4 2 1
//output q:1 2 4 5 8

2. greater的用法：

// greater example
#include <iostream>     // std::cout
#include <functional>   // std::greater
#include <algorithm>    // std::sort

int main () {
  int numbers[]={20,40,50,10,30};
  std::sort (numbers, numbers+5, std::greater<int>());
  for (int i=0; i<5; i++)
    std::cout << numbers[i] << ' ';
  std::cout << '
';
  return 0;
}
/************output: 50 40 30 20 10 *************/

 3. 在优先队列中使用greater: 却是从小到大排列顺序。

#include <queue>
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

class cmp
{
    bool reverse;
public:
    cmp(const bool &re = false)
    {    
        reverse = re;
    } 

    bool operator() (int &a, int &b) const
    {
        if(reverse) return (a > b);
        else return (a < b);
    }
};

int main()
{
    int myints[] = {10, 60, 50, 20};
    //默认从大到小 默认为less 
    priority_queue<int> second (myints, myints+4); //60, 50, 20, 10
    while(!second.empty())
    {
        cout << " " << second.top();
        second.pop();
    }
    cout << endl;
    /*从小到大: 10, 20, 50, 60*/
    priority_queue<int, vector<int>, greater<int> > third (myints, myints + 4);
    
    while(!third.empty())
    {
        cout << " " << third.top();
        third.pop();
    }
    cout << endl;
    /*从小到大, 10 ,20, 50, 60*/
    priority_queue<int, vector<int>, cmp> fourth(myints, myints+4, cmp(true));
    
    while(!fourth.empty())
    {
        cout << " " << fourth.top();
        fourth.pop();
    }
    cout << endl;

    return 0;
}