STL

C++-priority_queue

usage of priority_queue in STL

Posted by Haven Tong on 2019-11-18

C+±priority_queue

C++ STL-priority_queue 用法

priority_queue存在于头文件<queue>中,以下代码解释了该容器如何使用

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#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
#include <vector>

using namespace std;

struct Node{
int a,b;
Node(){}
Node(int aa,int bb):a(aa),b(bb){}

// 二级排序,a相同时按照b升序排序,否则按照a升序排序
// (a,b): (1,1)<(1,2)<(2,1)<(2,2)
// 这里重载的大于号和小于号在逻辑上等价
bool operator < (const Node &r) const{
if(a==r.a)
return b<r.b;
else
return a<r.a;
}
bool operator > (const Node &r) const{
if(a==r.a)
return b>r.b;
else
return a>r.a;
}
};

int main(){

//priority_queue 是基于堆实现的优先队列

// .size()返回当前优先队列的元素个数
// empty()为true表示当前队列为空,false表示不空
priority_queue<int> tmp_q;
cout<<tmp_q.size()<<endl; //0
cout<<tmp_q.empty()<<endl; //1
tmp_q.push(1);
cout<<tmp_q.size()<<endl; //1
cout<<tmp_q.empty()<<endl; //0
tmp_q.pop();
cout<<tmp_q.size()<<endl; //0
cout<<tmp_q.empty()<<endl; //1

// priority_queue<int> 表示队列中存储的元素是int类型(STL模板类)
// 默认priority_queue表示队列按照less出队
// 后出队的元素比先出队的元素要小,出队的元素越来越小,所以是less
// push 加入元素
// top 返回队首元素
// pop 队首元素出队
priority_queue<int> q;
q.push(5);
q.push(6);
cout<<q.top()<<endl; //6
q.pop();
cout<<q.top()<<endl; //5
q.pop();

// greater 表示后出队的元素比先出队的元素要大, 出队的元素越来越大,所以是greater
priority_queue<int, vector<int>, greater<int>> q_greater;
// <>中第二个参数vector<int>表示优先队列中使用vector<int> 存储元素,可以更换成deque
// 第二个参数中更换的其他类型应支持.begin(),.end(),.push(),.pop()等操作
// 绝大多数情况都使用vector<>,极少需要更换
// 例子:priority_queue<int, deque<int>, greater<int>> q_greater;
q_greater.push(5);
q_greater.push(6);
cout<<q_greater.top()<<endl; //5
q_greater.pop();
cout<<q_greater.top()<<endl; //6
q_greater.pop();

// 更换为double类型,出队法则是less
priority_queue<double, vector<double>, less<double>> q_less;
q_less.push(5.2);
q_less.push(6.6);
cout<<q_less.top()<<endl; //6.6
q_less.pop();
cout<<q_less.top()<<endl; //5.2
q_less.pop();

// 自定义类Node的priority_queue 使用greater需要对元素重载大于号
priority_queue<Node, vector<Node>, greater<Node>> q_greater_node;
q_greater_node.push(Node(1,1));
q_greater_node.push(Node(1,2));
q_greater_node.push(Node(2,1));
q_greater_node.push(Node(2,2));
cout<<q_greater_node.top().a<<" "<<q_greater_node.top().b<<endl; // 1 1
q_greater_node.pop();
cout<<q_greater_node.top().a<<" "<<q_greater_node.top().b<<endl; // 1 2
q_greater_node.pop();
cout<<q_greater_node.top().a<<" "<<q_greater_node.top().b<<endl; // 2 1
q_greater_node.pop();
cout<<q_greater_node.top().a<<" "<<q_greater_node.top().b<<endl; // 2 2
q_greater_node.pop();

// 自定义类Node的priority_queue 使用less需要对元素重载小于号
priority_queue<Node, vector<Node>, less<Node>> q_less_node;
q_less_node.push(Node(1,1));
q_less_node.push(Node(1,2));
q_less_node.push(Node(2,1));
q_less_node.push(Node(2,2));
cout<<q_less_node.top().a<<" "<<q_less_node.top().b<<endl; // 2 2
q_less_node.pop();
cout<<q_less_node.top().a<<" "<<q_less_node.top().b<<endl; // 2 1
q_less_node.pop();
cout<<q_less_node.top().a<<" "<<q_less_node.top().b<<endl; // 1 2
q_less_node.pop();
cout<<q_less_node.top().a<<" "<<q_less_node.top().b<<endl; // 1 1
q_less_node.pop();

return 0;
}