区间判断的map对端点覆盖问题

区间选点

区间问题一般对右端点进行排序并用右端点映射左端点
可以高效判断

wa

区间覆盖了，左端点不是最大值

#include<iostream>
#include<algorithm>
#include<map>

using namespace std;

int n;
map<int,int> alls;

int main(){
    int res = 0;
    int ed = -2e9;

    cin >> n;

    for(int i = 0;i < n;i ++){
        int a,b;
        cin >> a >> b;

        alls[b] = a;
    }

    for(auto i : alls){
        if(ed < i.second){
            res ++;
            ed = i.first;
        }
    }

    cout << res;
}

ac

multimap直接存所有

#include<iostream>
#include<algorithm>
#include<map>

using namespace std;

int n;
multimap<int,int> alls;

int main(){
    int res = 0;
    int ed = -2e9;

    cin >> n;

    for(int i = 0;i < n;i ++){
        int a,b;
        cin >> a >> b;

        alls.insert(pair<int,int>{b,a});  //pair<int,int>{b,a} == make_pair(b,a)
    }

    for(auto i : alls){
        if(ed < i.second){
            res ++;
            ed = i.first;
        }
    }

    cout << res;
}

或者map + 重合判断

#include<iostream>
#include<algorithm>
#include<map>

using namespace std;

int n;
map<int,int> alls;

int main(){
    int res = 0;
    int ed = -2e9;

    cin >> n;

    for(int i = 0;i < n;i ++){
        int a,b;
        cin >> a >> b;

        if(alls.find(b) != alls.end()) a = max(a,alls[b]);
        alls[b] = a;
    }

    for(auto i : alls){
        if(ed < i.second){
            res ++;
            ed = i.first;
        }
    }

    cout << res;
}