UVa355 - The Bases Are Loaded

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <stack>
using namespace std;
int change1(char a)
{
if(a>='0' && a<='9')
return a-'0';
else
return a-'A'+10;
}
void solve(unsigned long long n, int k)
{
stack<int> ans;
int temp;
if(n==0) {
printf("0");
return;
}
while(n) {
ans.push(n%k);
n /= k;
}
while(!ans.empty()) {
temp = ans.top();
ans.pop();
if(temp<=9)
printf("%d",temp);
else
printf("%c",temp-10+'A');
}
}
int main()
{
int i, b1, b2, len, t;
char str[15];
bool flag;
unsigned long long sum, k;
while(cin>>b1>>b2) {
cin>>str;
len = strlen(str);
sum = 0;
k=1;
flag = true;
for(i=len-1; i>=0; i--) {
t = change1(str[i]);
if( (t >= b1 || t < 0) || str[i]=='-') {
cout << str << " is an illegal base " << b1 << " number" << endl;
flag = false;
break;
}
sum += t*k;
k *= b1;
}
if(flag) {
cout << str << " base " << b1 <<" = ";
solve(sum, b2);
cout << " base " << b2 << endl;
}
}
return 0;
}