全WA，求助

#include <iostream>
#include <cstring>
using namespace std;
string s1,s2,a="",b="",ans="";
int c,tmp=0;
int main(){
    cin>>s1>>s2;
    for (int i=s1.length()-1;i>=0;i--) a+=s1[i];
    for (int i=s2.length()-1;i>=0;i--) a+=s2[i];
    for (int i=0;i<max(a.length(),b.length());i++){
        if(i>=a.length()) a+='0';
        if(i>=b.length()) b+='0';
        c=(a[i]-'0')+(b[i]-'0')+tmp;
        if(c>=10){
            tmp=c/10;
            c %=10;
        }
        else tmp=0;
        ans+=c+'0';
    }
    if(tmp !=0) ans += tmp + '0';
    for(int i=ans.length()-1;i>=0;i--) cout <<ans[i];
    return 0;
}