神犇看一下怎么回事

源代码

//#include <bits/stdc++.h>
#include <algorithm>
#include <string>
#include<iostream>
using namespace std;

bool is_huiwen(const string &str) {
	int len = str.length();
	for (int i = 0; i < len / 2; i++) {
		if (str[i] != str[len - i - 1]) {
			return false;
		}
	}
	return true;
}

string reverse_str(string str) {
	reverse(str.begin(), str.end());
	return str;
}

unsigned int string2num(const string str, int scale) {
	unsigned int result = 0;
	for (int i = 0; i < str.length(); i++) {
		result *= scale;
		if (str[i] >= '0' && str[i] <= '9')
			result += str[i] - '0';
		else {
			result += str[i] - 'A' + 10;
		}
	}
	return result;
}

string num2string(unsigned int num, int scale) {
	string str = "";
	while (num) {
		int rem = num % scale;
		num /= scale;
		if (rem >= 0 && rem <= 9) {
			str += rem + '0';
		}
		else {
			str += rem + 'A' - 10;
		}
	}
	return reverse_str(str);
}

string plus_str(const string &num1, const string &num2, int scale) {
	//cout << "num1: " << string2num(num1, scale) << ", num2: " << string2num(num2, scale) << endl;
	unsigned int num = string2num(num1, scale) + string2num(num2, scale);
	return num2string(num, scale);
}



int main() {
	int scale, step = 0;
	string number;
	cin >> scale >> number;
	//string str = num2string(number, scale);
	string str = number;
	while (step <= 30) {
		step++;
		str = plus_str(str, reverse_str(str), scale);
		//cout << str << endl;
		if (is_huiwen(str)) {
			cout << "STEP=" << step;
			break;
		}
	}
	if (step > 30) {
		cout << "Impossible!";
	}
}

第三个WA了，其他AC

帮忙看一眼哪里错了