PAT---1024 Palindromic Number(大数加法+回文)

#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

string sum(string a, string b)
{
    if(a.size() < b.size())
    {
        string tmp = b;
        b = a;
        a = tmp;
    }
    int len = a.size() - b.size();
    for(int i = 0; i < len; i++)
        b = '0' + b;
    bool flag = false;
    string ans;
    for(int i = a.size()-1; i >= 0; i--)
    {
        int tmp = a[i]+b[i]-'0'-'0'+flag;
        if(tmp >= 10)
        {
            tmp -= 10;
            flag = true;
        }
        else
            flag = false;
        ans = (char)(tmp+'0')+ans;
    }
    if(flag)
        ans = '1' + ans;
    return  ans;
}

bool isPal(string str)
{
    for(int i = 0; i < str.size()/2; i++)
        if(str[i] != str[str.size()-1-i])
            return false;
    return true;
}

int main()
{
    string str, restr;
    int k;
    cin >> str >> k;
    if(isPal(str))
    {
         cout << str << endl;
         cout << "0" << endl;
    }
    else
    {
        restr = str;
        reverse(restr.begin(), restr.end());
        string tmp;
        int i;
        for(i = 1; i <= k; i++)
        {
            tmp = sum(str, restr);
            if(isPal(tmp))
                break;
            restr = str = tmp;
            reverse(restr.begin(), restr.end());
        }
        cout << tmp << endl;
        if(i > k)
            cout << k << endl;
        else
            cout << i << endl;
    }
    return 0;
}