相信我 这是袁老师的题解

#include<bits/stdc++.h>
using namespace std;

int main()
{
int n;
cin >> n;

string d,f;	 
vector <string> v; 
while(n--)
{
	cin >> d;
	if(d=="ADD")
	{
		cin >> f; 
		if( find(v.begin(), v.end(), f) == v.end()) 
		{
			v.push_back(f);
			cout << "success\n";
		}else{
			cout << "already exists\n";
		}
	}

	if(d=="DEL")
	{
		cin >> f;
		if ( find(v.begin(), v.end(), f) == v.end() )
		{
			cout << "non-existent\n";
		}else{
			v.erase(  find(v.begin(), v.end(), f)  );
			cout << "success\n";
		}
	}
	
	if(d=="VIEW")
	{
		if(v.size()==0)
			cout << "empty\n";
		else{
			for(int i=0; i<v.size(); i++)
				cout << i+1 << ". " << v[i] << endl;
		}
	}

	if(d=="UNLOCK")
	{
		cin >> f;
		if ( find(v.begin(), v.end(), f) == v.end() )  
			cout << "NO\n";
		else
			cout << "YES\n";
	}
}
return 0;

}