I coudn't get input into a global string

      #include<bits/stdc++.h>
      using namespace std;
      string s1,s2;
      int lcs(int i,int j)
       {
                  if(s1[i]=='\0' || s2[j]=='\0')
                  {
                         return(0);
                  }
                  else if(s1[i]==s2[j])
                  {
                         return 1+lcs(i+1,j+1);
                  }
                  else
                  {
                         return max(lcs(i+1,j),lcs(i,j+1));
                  }
       }
       int main()
       {
             cin>>s1>>s2;
             cout<<lcs(0,0);
             return(0);
       }

In this code after I execute and give input the output is always 0 this is because the cin statement is not reassigning the strings "s1" and "s2" why is this happening?