前三个全wa，求求大佬!

#include <cstdio>
#include <cstring>
#include <iostream>
int main()
{
    int n;
    char s[100005];
    scanf("%d %s", &n, s);
    int j = 0,ls = strlen(s);
    int sum = 0;
    int a[100005], b[100005];
    for (int i = 1; i <= ls; i++) {
        if (s[i] <= 9 && s[i] >= 0)
            a[i] = s[i] - '0';
        else a[i] = s[i] - 'A';//十六进制
    }
    for (int i = 1; i <= ls; i++) {//倒转
        b[i] = a[ls +1 -i];
    }
    for(int i = 1;i <= ls;i++) {
        if (b == a)
            printf("STEP=%d", sum);
        else {
            b[i] += a[i];
            b[i + 1] += a[i] / n;
            b[i] %= n;

            sum++;
        }
        if (b[ls + 1] > 0) 
            ls++;
    }
    printf("Impossible!");
    return 0;
}