Why are we able to compare vector element with string element without initializing the vector? How does it work? :     #include<bits/stdc++.h>
    using namespace std;
     
    int main()
`   {
      int t;
      cin>>t;
      while(t--)
      {
        string x; cin>>x;
        vector<char> a;
        for(int i=0;i<x.length();i++)
        {
         int f=0;
         for(int j=0;j<a.size();j++)
          {
            if(a[j]==x.at(i))
               {
                f=1;
                break;
               }
          }
          if(f==0)
             a.push_back(x.at(i));
        }
        if(a.size()==2)
           cout<<"YES\n";
        else 
           cout<<"NO\n";
      }
      return 0;
    }