rt，如果我这么写:

#include<bits/stdc++.h>
using namespace std;
map<int, map<int,int> >Line;
map<int,int> K;
int cnt;
signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n;
	cin >> n;
	for(int i = 1;i <= n;++i){
		int oper,k,b;
		cin >> oper >> k >> b;
		if(oper == 1)++cnt,++K[k],++Line[k][b];    
		if(oper == 2) cout << cnt - K[k] << '\n';
		if(oper == 3) {
			for(map<int,int>::iterator it = K.begin();it != K.end();++it){
				if(it -> first == k)cnt -= Line[k][b],K[k] -= Line[k][b],Line[k][b] = 0;
				else cnt -= K[it -> first],Line[k][b] = K[k] = 0;
			}
		}
	}
	return 0;
}

样例都过不了
但是如果这样写：

#include<bits/stdc++.h>
using namespace std;
map<int, map<int,int> >Line;
map<int,int> K;
int cnt;
signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n;
	cin >> n;
	for(int i = 1;i <= n;++i){
		int oper,k,b;
		cin >> oper >> k >> b;
		if(oper == 1)++cnt,++K[k],++Line[k][b];    
		if(oper == 2) cout << cnt - K[k] << '\n';
		if(oper == 3) {
			for(map<int,int>::iterator it = K.begin();it != K.end();){
				if(it -> first == k){
					cnt -= Line[k][b],K[k] -= Line[k][b];
					Line[k].erase(b);
					++it;
					if(!K[k]){K.erase(k),Line.erase(k);}
				}
				else {
					int tmp = it -> first;
					++it;
					cnt -= K[tmp];
					K.erase(tmp),Line.erase(tmp);
				}
			}
			if(!K[k]){K.erase(k),Line.erase(k);}
		}
	}
	return 0;
}

100pts，请问这是为什么