STL: set 和 priority_queue

1. 优先级队列默认是底层为vector实现的heap
2. set 基于平衡红黑树
3. 两者插入元素时间复杂度都是O(log(n))
4. 使用默认的greater 得到的结果不同

#include <iostream>#include <queue>#include <vector>#include <set>int main(){    using namespace std;    vector<int>vec{1,3,5,78,4,23};    set<int, greater<int> > myset;    priority_queue<int, vector<int>, greater<int> >myque;    for(int i=0; i<6; ++i){        myset.insert(vec[i]);         myque.push(vec[i]);             }    for(set<int, greater<int> >::iterator it=myset.begin() ; it!=myset.end(); ++it){        cout << *it <<" ";       }//从大到小：78 23 5 4 3 1    cout << endl;    for(int i=0; i<6; ++i){        cout << myque.top() <<" ";          myque.pop();            }// 小顶堆： 1 3 4 5 23 78    getchar();    getchar();    return 0;}

优先级队列、sort、qsort 与 重载、自定义cmp函数的关系

#include <iostream>#include <string>#include <algorithm>   // for sort()#include <queue>       // for priority_queue#include <map>#include <set>using namespace std;/*************************************************************************/// 重载 < 运算class student_cmp{private:    string name;    int id;public:    student_cmp(string name = "default", int id = 0) :        name(name) ,        id(id)    {        ;    }    inline        string get_name(){        return name;    }    inline        int get_id(){        return id;    }    // priority_queue: id 大的优先级低， 从小到大排列  == greater<int>    // vector sort 中 从大到小排    bool operator < (const student_cmp &s)const{        return this->id > s.id;    }    friend ostream &operator << (ostream & os, const student_cmp &s){        os << s.name << ": " << s.id << endl;        return os;    }};/**********************************************************************/// 没有重载< 运算， 使用自定义cmp函数class student_nocmp{private:    string name;    int id;public:    student_nocmp(string name = "default", int id = 0) :        name(name),        id(id)    {        ;    }    inline     string get_name(){        return name;    }    inline    int get_id(){        return id;    }    friend ostream &operator << (ostream & os, const student_nocmp &s){        os << s.name << ": " << s.id << endl;        return os;    }};// 优先级队列：从小到大排列class mynocmp{public:    bool operator ()(student_nocmp &a, student_nocmp &b)    {        return a.get_id() > b.get_id();    }};// for sort  必须返回bool类型// vector: 从大到小排列bool sort_nocmp (student_nocmp &a, student_nocmp &b){    return a.get_id() > b.get_id();}// for qsort  必须返回int类型, 参数是指针类型// array: 从大到小排列// 如果返回值小于0，则qsort 就得知第一个元素应该在前，如果返回值大于0，则第一个元素应该在后。// 相减顺序与参数一致就是从小到大排序int qsort_nocmp(const void *a, const void *b){    student_nocmp * p1, *p2;    p1 = (student_nocmp *)a;    p2 = (student_nocmp *)b;    return p1->get_id() - p2->get_id();}/**********************************************************************/int main(){    cout << "overload operator < in priority_queue..." << endl;    priority_queue<student_cmp> stuQueue;    stuQueue.push(student_cmp());    stuQueue.push(student_cmp("zhangsan", 23));    stuQueue.push(student_cmp("lisi", 14));    stuQueue.push(student_cmp("laowang", 36));    stuQueue.push(student_cmp("xiaoming", 28));    while (!stuQueue.empty()){        cout << stuQueue.top();        stuQueue.pop();    }    cout << endl;    cout << "without overload operator < in priority_queue..." << endl;    priority_queue<student_nocmp, vector<student_nocmp>, mynocmp> stunoQueue;    stunoQueue.push(student_nocmp());    stunoQueue.push(student_nocmp("zhangsan", 23));    stunoQueue.push(student_nocmp("lisi", 14));    stunoQueue.push(student_nocmp("laowang", 36));    stunoQueue.push(student_nocmp("xiaoming", 28));    while (!stunoQueue.empty()){        cout << stunoQueue.top();        stunoQueue.pop();    }    cout << endl;    /******sort******************/    cout << "overload operator < in vector..." << endl;    vector<student_cmp> veccmp;    veccmp.push_back(student_cmp());    veccmp.push_back(student_cmp("zhangsan", 23));    veccmp.push_back(student_cmp("lisi", 14));    veccmp.push_back(student_cmp("laowang", 36));    veccmp.push_back(student_cmp("xiaoming", 28));    sort(veccmp.begin(), veccmp.end());    for (vector<student_cmp>::iterator it = veccmp.begin(); it != veccmp.end(); ++it)        cout << *it;    cout << endl;    cout << "without overload operator < in vector..." << endl;    vector<student_nocmp> vecnocmp;    vecnocmp.push_back(student_nocmp());    vecnocmp.push_back(student_nocmp("zhangsan", 23));    vecnocmp.push_back(student_nocmp("lisi", 14));    vecnocmp.push_back(student_nocmp("laowang", 36));    vecnocmp.push_back(student_nocmp("xiaoming", 28));    sort(vecnocmp.begin(), vecnocmp.end(), &sort_nocmp);    for (vector<student_nocmp>::iterator it = vecnocmp.begin(); it != vecnocmp.end(); ++it)        cout << *it;    cout << endl;    cout << "without overload operator < in array..." << endl;    student_nocmp arrnocmp[5] = { student_nocmp(),                                  student_nocmp("zhangsan", 23),                                 student_nocmp("lisi", 14),                                 student_nocmp("laowang", 36),                                 student_nocmp("xiaoming", 28)    };    qsort(arrnocmp, 5, sizeof(student_nocmp), &qsort_nocmp);    for (int i = 0; i < 5; ++i)        cout << arrnocmp[i];    getchar();    return 0;}

---

结果：

---

结论：
1. 优先级队列定义的< 运算 规定低优先级， 返回为ture的条件说明优先级低；
2. 类外部定义的cmp相当于实现重载operator <;
3. 由于qsort返回值是int型， /如果返回值小于0，则qsort 就得知第一个元素应该在前，如果返回值大于0，则第一个元素应该在后。即，相减顺序与参数一致就是从小到大排序；
4. sort 的效率比qsort高， 还是使用sort 就好了；