string swap(string s1, string s2, string s)
{
    for (int i=0;i<s.length();i++)
    {
        if (s[i]==s1[0])
        {
            bool b = true;
            int j=0;
            int i1=i;
            for (j=0;(j<s1.length() && i1<s.length());i1++,j++)
            {
                if (s[i1]!=s1[j])
                    b=false;
            }
            if (b && j==s1.length())
            {
                    for (int k=0;k<s1.length();k++,i++)
                {
                    s[i]=s2[k];
                }
                i--;
            }
        }
    }
    return s;
}