runtime error: applying non-zero offset 18446744073709551615 to null pointer (basic_string.h)

Question

I am solving a question on leetcode where I have to multiply strings and I got this error which I cant understand

Line 518: Char 69: runtime error: applying non-zero offset 18446744073709551615 to null pointer (basic_string.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/basic_string.h:527:69

Here is my code

string multiply(string s1, string s2) {
         vector<int> vec(s1.length()+s2.length(),0);
       for(int i=s2.length()-1;i>=0;i--){
           for(int j=s1.length()-1;j>=0;j--){
               int pro = (s1[j]-'0')*(s2[i]-'0') ;
               //borrow sent to next
               int sum = vec[i+j+1] + pro;
                vec[i+j+1] = (sum%10);
                vec[i+j] += (sum/10);
           }
       }
       string ans="";
       for(int i=0; i<(s1.length()+s2.length());i++){
           if(ans=="" && vec[i]==0){
               continue;
           }
           ans.push_back(vec[i]+'0');
       }
       if(ans==""){
           return 0;
       }
       return ans;

    }

LINK : https://leetcode.com/problems/multiply-strings/

Answer 1

For the case where ans="",
Instead of returning "0" (string), you are returning 0 (Integer).
As the return type is a string it is causing a problem.

if(ans=="")
{
   return "0";
}

This Should work fine, I think.