呵呵

已找到第二组测试数据(十六进制):
第二组数据：
16
AC27
附改正后代码:

#include <iostream>
#include <cctype>
#include <algorithm>
#include <cstring>
#include <sstream>
using namespace std;

int convert1(char ch) {
    if (isdigit(ch))
        return ch - '0';
    else
        return 10 + 'A' - ch;
}

char convert2(int n) {
    if (0 <= n && n <= 9)
        return '0' + n;
    else
        return 'A' + n - 10;
}

string add(string s1, string s2, int base) {
    if (s1.length() < s2.length())
        swap(s1, s2);
    int maxsize = max(s1.length(), s2.length()) + 1;
    int a[s1.length()], b[s2.length()], c[maxsize];
    for (int i = 0; i < s1.length(); i++) a[i] = convert1(s1[s1.length() - i - 1]);
    for (int i = 0; i < s2.length(); i++) b[i] = convert1(s2[s2.length() - i - 1]);
    memset(c, 0, sizeof(c));
    for (int i = 0; i < s1.length(); i++) {
        c[i] += a[i];
        if (i < s2.length())
            c[i] += b[i];
        c[i + 1] += c[i] / base;
        c[i] %= base;
    }
    string result = "";
    for (int i = maxsize - 1; i >= 0; i--) {
        if (i == maxsize - 1 && convert2(c[i]) == '0') continue;
        result.push_back(convert2(c[i]));
    }
    return result;
}

int main() {
    int base;
    cin >> base;
    string num;
    cin >> num;
    bool can = false;
    for (int i = 1; i <= 30; i++) {
        string tmp = num;
        reverse(tmp.begin(), tmp.end());
        num = add(tmp, num, base);
        tmp = num;
        reverse(tmp.begin(), tmp.end());
        if (num == tmp) {
            cout << "STEP=" << i << endl;
            can = true;
            break;
        }
    }
    if (!can)
        cout << "Impossible!" << endl;
    return 0;
}