回文数 (Palindrome Numbers,Dhaka 2003,LA 2889)

#include <iostream>
#include <string.h>
#include <string>
#include <fstream>
#include <algorithm>
#include <stdio.h>
#include <vector>
#include <queue>
#include <set>
#include <cmath>
using namespace std;
const double eps = 1e-8;
const int INF=0x7fffffff;
unsigned long long uINF = ~0LL;
#define MAXN 10000007
typedef long long LL;
LL vis[MAXN];
LL prime[MAXN];

void sieve(LL n)
{
    LL m=(LL)sqrt(n+0.5);
    memset(vis,0,sizeof(vis));
    for(LL i=2;i<=m;i++)if(!vis[i])
    for(LL j=i*i;j<=n;j+=i)vis[j]=1;
}

LL gen_prime(LL n)
{
    sieve(n);
    LL c=0;
    for(LL i=2;i<=n;i++)if(!vis[i])
        prime[c++]=i;
    return c;
}

LL gcd(LL a,LL b)
{
    return b==0?a:gcd(b,a%b);
}
LL num[11];
LL ex[11];
void init()
{
    num[0]=1;ex[0]=0;
    num[1]=9;ex[1]=1;
    for(int i=2;i<=10;i++)
    {num[i]=num[i-1]*10;
    ex[i]=ex[i-1]*10;}

    //for(int i=1;i<11;i++)
   //cout<<num[i]<<' ';
}

string Reverse(string str1)
{
    string str2="";
    for(int i=str1.length()-1;i>=0;i--)
    str2+=str1[i];
    return str2;
}

int main()
{
    LL n;
    init();
    while(scanf("%lld",&n),n)
    {
        int i;
        string ans="";
        bool odds;
        LL dig=1;
        for(i=1;i<11;i++)
        {
            if(n>num[i])n-=num[i];
            else {odds=true;break;}
            if(n>num[i])n-=num[i];
            else {odds=false;break;}
            dig*=10;
        }
        n--;n+=dig;
        while(n>0)
        {
            ans+=n%10+'0';
            n/=10;
        }
        ans=Reverse(ans);
        if(odds)ans+=Reverse(ans.substr(0,ans.length()-1));
        else ans+=Reverse(ans);
        cout<<ans<<endl;
    }

    return 0;
}