leetcode 415 两个字符串相加

string addstring(string s1,string s2)
{
    string ans="";
    int f=;
    for(int i=s1.length()-,j=s2.length()-;i>=||j>=;i--,j--)
    {
        int x=i<?:s1[i]-'';
        int y=j<?:s2[j]-'';
        char c=(x+y+f)%+'';
        ans=c+ans;
        f=(x+y+f)/;
    }
    if(f==)
        ans=''+ans;
    return ans;
}