UVa 424 Integer Inquiry Solution

#include<iostream>
#include<string>
#include<cstring>
#include<sstream>
#include<cctype>
#include<string.h>
#include<algorithm>
#include<cmath>
#include<stack>
#include<fstream>
#include<cstdlib>
#include<iomanip>
#include<vector>
#include<map>
#include<set>
using namespace std;

string rev(string a){
                    int i,j;
                    string b="";
                    for(i=0,j=a.length()-1;i<a.length();i++,j--)
                            b+=a[j];
                    return b;
                    }
string add(string a, string b){
                                int i,j,x,y,sum,s,temp=0;
                                string c="";
                                string e;
                                if(a.length()<b.length()){
                                                         e=a;
                                                         a=b;
                                                         b=e;
                                                         }
                                for(i=0;i<a.length();i++)
                                    {
                                    if(i<b.length())
                                            {
                                            x=a[i]-'0';
                                            y=b[i]-'0';
                                            sum=x+y+temp;
                                            s=sum%10;
                                            c+=s+'0';
                                            temp=0;
                                            if(sum>9) temp=1;
                                            if(i==a.length()-1 && temp==1)
                                                c+='1';
                                            }
                                    else {
                                         x=a[i]-'0';
                                         sum=temp+x;
                                         s=sum%10;
                                         c+=s+'0';
                                         temp=0;
                                         if(sum>9) temp=1;
                                            if(i==a.length()-1 && temp==1)
                                                c+='1';
                                         }
                                    }
                                return c;
                                }

int main()
{
string sum="0",s,a;
while(cin>>s)
    {
    if(s=="0") break;
    a=rev(s);
    sum=add(a,sum);
    }
cout<<rev(sum)<<endl;
return 0;
}