UVa 495 Fibonacci Freeze 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 e, string f){
                                int i,j,x,y,sum,s;
                                string c="";
                                string a,b,d;
                                int temp=0;
                                a=rev(e);
                                b=rev(f);
                                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';
                                         }
                                    }
                                d=rev(c);
                                return d;
                                }

int main()
{
int i,x;
string a,b,e,f,c;
map<int,string>nt;
nt[0]="0";
nt[1]="1";
for(i=2;i<=5000;i++)
    nt[i]=add(nt[i-1],nt[i-2]);
while(cin>>x)
    {
    cout<<"The Fibonacci number for "<<x<<" is "<<nt[x]<<endl;
    }
return 0;
}