优先队列

寻找队列中的最大值或最小值，出队。

优先队列的底层实现：堆；（对于堆的底层实现，面试时经常会出）。

C++中的优先队列容器：priority_queue

1. 默认从大到小排列：（最大堆）

#include <iostream>
#include <queue>
#include <ctime>
using namespace std;

int main() {
    srand(time(NULL));     //产生随机数
    priority_queue<int> pq;
    for (int i = 0; i < 10;i++) {
        int num = rand() % 100;
        pq.push(num);
        cout << "insert" << num << "in priority queue." << endl;

    }
    while (!pq.empty()) {
        cout << pq.top() << " ";
        pq.pop();
    }
    return 0;
}

2. 小顶堆

#include <iostream>
#include <queue>
#include <ctime>
using namespace std;

int main() {
    srand(time(NULL));     //产生随机数
    priority_queue<int, vector<int>, greater<int> > pq2;   //最小堆，传入为int型，底层数据结构实现一般为vector，比较函数greater

    for (int i = 0; i < 10;i++) {
        int num = rand() % 100;
        pq2.push(num);
        cout << "insert" << num << "in priority queue." << endl;
    }

    while (!pq2.empty()) {
        cout << pq2.top() << " ";
        pq2.pop();
    }
    return 0;
}

3. 自定义比较函数：

#include <iostream>
#include <queue>
#include <ctime>
using namespace std;

struct myCmp
{
    bool operator()(int a, int b){
        return a % 10 < b % 10;   //比较个位数，个位数越大越靠前
    }
};

int main() {
    srand(time(NULL));     //产生随机数
    priority_queue<int> pq;
    for (int i = 0; i < 10;i++) {
        int num = rand() % 100;
        pq.push(num);
        cout << "insert" << num << "in priority queue." << endl;

    }
    while (!pq.empty()) {
        cout << pq.top() << " ";
        pq.pop();
    }
    cout << endl << endl;

    priority_queue<int, vector<int>, greater<int> > pq2;   //底层是最小堆

    for (int i = 0; i < 10;i++) {
        int num = rand() % 100;
        pq2.push(num);
        cout << "insert" << num << "in priority queue." << endl;
    }

    while (!pq2.empty()) {
        cout << pq2.top() << " ";
        pq2.pop();
    }

    //使用自定义Comparator的priority queue
    priority_queue<int, vector<int>, myCmp> pq3;

    for (int i = 0; i < 10;i++) {
        int num = rand() % 100;
        pq3.push(num);
        cout << "insert" << num << "in priority queue." << endl;
    }

    while (!pq3.empty()) {
        cout << pq3.top() << " ";
        pq3.pop();
    }

    return 0;
}