题解

直接拿map模拟

#include <iostream>
#include <cstdio>
#include <map>
using namespace std;
map <int, int> mp;
int main(){
    int t;
    cin >> t;
    while(t--){
        int tmp, val;
        cin >> tmp;
        if(tmp == 1) cin >> val, mp[val]++;//增加
        if(tmp == 2){
            cin >> val;
            if(--mp[val] == 0) mp.erase(val);//删除
        }
        if(tmp == 3) cout << mp.size() << endl;//输出
    }

	return 0;
}