Priority Queues优先级队列详解

概述

STL中 class priority_queue<>实作出一个queue，其中的元素根据优先级被读取，top() / pop()移除/存取下一个元素。这里的“下一个元素”是指“优先级最高”的元素。即priority_queue内的元素已经根据其值进行了排序。

在priority_queue和queue都定义于头文件<queue>

namespace std{template <class T,          class Container = vector<T> ,  Class Compare = less<tyname Container::value_type> >Class priority_queue;}

第一个template参数是元素类型，第二个template参数定义了内部元素实际存放的容器，默认是vector，但必须要支持存取随机迭代器。第三个template参数是“用以搜寻下一个最高优先级元素”的排序准则。默认是以operator<作为比较标准,图1展示了priority_queue的接口。

核心接口

priority_queue的核心接口主要由成员函数push(),top(),pop()组成：

push()

• 将一个元素置入priority_queue内.

pop()

• 移除priority_queue中“优先级最高”的元素，如果存在多个相等的元素，则无法确知会移除哪一个.
• 调用者保证priority_queue非空.
• 该函数无返回值，想处理被移除的元素，需要调用top().

top()

• 从priority_queue中返回“优先级最高的元素”，如果存在多个相等的元素，则无法确知会返回哪一个.
• 调用者必须保证priority_queue非空，否则会导致未定义行为.

源码实现

template<class _Ty,class _Container = vector<_Ty>,class _Pr = less<typename _Container::value_type> >class priority_queue{// priority queue implemented with a _Containerpublic:typedef _Container container_type;typedef typename _Container::value_type value_type;typedef typename _Container::size_type size_type;typedef typename _Container::reference reference;typedef typename _Container::const_reference const_reference;priority_queue(): c(), comp(){// construct with empty container, default comparator}explicit priority_queue(const _Pr& _Pred): c(), comp(_Pred){// construct with empty container, specified comparator}priority_queue(const _Pr& _Pred, const _Container& _Cont): c(_Cont), comp(_Pred){// construct by copying specified container, comparatormake_heap(c.begin(), c.end(), comp);}template<class _Iter>priority_queue(_Iter _First, _Iter _Last): c(_First, _Last), comp(){// construct by copying [_First, _Last), default comparatormake_heap(c.begin(), c.end(), comp);}template<class _Iter>priority_queue(_Iter _First, _Iter _Last, const _Pr& _Pred): c(_First, _Last), comp(_Pred){// construct by copying [_First, _Last), specified comparatormake_heap(c.begin(), c.end(), comp);}template<class _Iter>priority_queue(_Iter _First, _Iter _Last, const _Pr& _Pred,const _Container& _Cont): c(_Cont), comp(_Pred){// construct by copying [_First, _Last), container, and comparatorc.insert(c.end(), _First, _Last);make_heap(c.begin(), c.end(), comp);}bool empty() const{// test if queue is emptyreturn (c.empty());}size_type size() const{// return length of queuereturn (c.size());}const_reference top() const{// return highest-priority elementreturn (c.front());}reference top(){// return mutable highest-priority element (retained)return (c.front());}void push(const value_type& _Pred){// insert value in priority orderc.push_back(_Pred);push_heap(c.begin(), c.end(), comp);}void pop(){// erase highest-priority elementpop_heap(c.begin(), c.end(), comp);c.pop_back();}protected:_Container c;// the underlying container_Pr comp;// the comparator functor};

应用实例

/*****************************************************************函数名称：PriorityQueuesTest*功    能：优先级队列使用说明*作    者：Jin*日    期：2016年7月6日****************************************************************/void PriorityQueuesTest(){    priority_queue<int> PriorityQue;        //insert elements into priority queue    PriorityQue.push(66);    PriorityQue.push(22);    PriorityQue.push(44);    cout <<"insert num to priority queue : 66 22 44" << endl;    //read and print two elements    cout <<"read first element and pop it: " << PriorityQue.top() << endl;    PriorityQue.pop();    cout << "read second element and pop it: " << PriorityQue.top() << endl;    PriorityQue.pop();    //insert three more elements    PriorityQue.push(11);    PriorityQue.push(55);    PriorityQue.push(33);    cout <<"insert num to priority queue : 11 55 33" << endl;        //pop one elements without dealing data    cout << "pop one elements" << endl;    PriorityQue.pop();    //pop and print remaining element    cout << "pop and print remaining element:" << endl;    while (!PriorityQue.empty())    {        cout << PriorityQue.top() << ' ';        PriorityQue.pop();    }    cout << endl;}

运行结果: