set集合

• set一种容器,用于储存唯一的元素（不允许重复），并按照升序排序(<)
• 内部实现是红黑树
  
  

比较函数重置

1. greater函数

#include<bits/stdc++.h>
using namespace std;
int main()
{
    set<int,greater<int>> mySet;
    mySet.insert(25);
    mySet.insert(17);
    mySet.insert(39);
    mySet.insert(42);

    for(const auto& elem : mySet){
        cout << elem <<" ";
    }
    cout << endl;

    return 0;
}

2. 自定义比较函数

#include<bits/stdc++.h>
using namespace std;
struct MyCompare{
    bool operator()(const int& a,const int& b)const{
        return a > b;
    }

};
int main()
{
    set<int,MyCompare> mySet;
    mySet.insert(25);
    mySet.insert(17);
    mySet.insert(39);
    mySet.insert(42);

    for (const auto& elem : mySet){
        cout << elem <<" ";
    }
    cout << endl;
    return 0;
}

multiset多重集合*

• 与set不同允许重复元素
  
  
• 其中erase()会删除相同的所有元素
• st.erase(st.find())只会删除相同的一个元素

unordered_set无序集合

• 没有特定的顺序
• 实现基础是哈希表
• 即插入，删除和查找的时间复杂度都是常数时间，O(1)
  

代码示例

1. set

#include<bits/stdc++.h>
using namespace std;
int main()
{
    set<int> myset;//声明set
    //插入元素
    myset.insert(5);
    myset.insert(2);
    myset.insert(8);
    myset.insert(2);
    cout << "Set elements: " ;
    for(const auto& elem : myset){
        cout << elem << " ";
    }
    cout << endl;
    int searchValue = 5;//搜索5
    auto it = myset.find(searchValue);
    if(it != myset.end()){
        cout << searchValue << " found in the set." << endl;

    }else{
        cout << searchValue << "not found in the set." << endl;

    }
    int removeValue = 2;//移除2
    myset.erase(removeValue);
    cout << "set elements after removal";
    for(const auto& elem : myset){
        cout << elem <<" ";

    }cout <<endl;
    myset.clear();//清空set
    if(myset.empty())//判断是否为空
    {
        cout << " set is empty" << endl;
    }else{
        cout << "set is not empty" << endl;
    }
    return 0;

}

2. multiset多重集合

#include<bits/stdc++.h>
using namespace std;
int main()
{
    multiset<int> myMultiset;
    myMultiset.insert(5);
    myMultiset.insert(2);
    myMultiset.insert(8);
    myMultiset.insert(2);

    cout << "Multiset elements:";
    for(const auto& elem : myMultiset){
        cout << elem <<" ";
    }cout << endl;
    int searchValue = 5;
    auto range = myMultiset.equal_range(searchValue);
    if(range.first != range.second){
        cout << searchValue <<" found in the multiset."<< endl;
    }else{
        cout << searchValue << " not found in the multiset."<< endl;

    }
    int removeValue = 2;
    myMultiset.erase(removeValue);
    cout << "Multiset elements after removal:";
    for (const auto& elem : myMultiset){
        cout << elem <<" ";

    }cout << endl;
    myMultiset.clear();
    if(myMultiset.empty()){
        cout << "Multiset is empty."<< endl;
    }else{
        cout << "Multiset is not empty." << endl;
    }
    return 0;
}

3. unordered_set无序集合

#include<bits/stdc++.h>
using namespace std;
int main()
{
    unordered_set<int> myUnorderedset;

    myUnorderedset.insert(5);
    myUnorderedset.insert(5);
    myUnorderedset.insert(5);
    myUnorderedset.insert(5);
    cout << "Unordered set elements:" ;
    for (const auto& elem : myUnorderedset){
        cout << elem <<" ";
    }cout << endl;
    int searchValue = 5;
    auto it = myUnorderedset.find(searchValue);
    if(it != myUnorderedset.end()){
        cout << searchValue << "found in the unordered set."<< endl;
    }else{
        cout << searchValue << "not found in the unordered set."<<endl;

    }
    int removeValue = 2;
    myUnorderedset.erase(removeValue);
    cout << "Unordered set elements after removal:";
    for(const auto& elem : myUnorderedset){
        cout << elem <<" ";

    }cout << endl;
    myUnorderedset.clear();
    if(myUnorderedset.empty()){
        cout << "Unordered set is empty."<< endl;
    }else{
        cout << "Unordered set is not empty."<< endl;
    }
    return 0;
}