仅#4 AC，求助 （自测用例全部正确，找不到出错的测试用例）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int a[200];
int n;

void highplusecho(){    //高精加（正+反）
    int b[200];
    for (int i = 1;i<=a[0];i++){
        b[i]=a[a[0]-i+1];   //转存反数
    }
    for (int i = 1;i<=a[0];i++){
        a[i]+=b[i];         //获取回文
    }
    for (int i = 1;i<=a[0];i++){
        a[i+1]+=a[i]/n;     //进位
        a[i]%=n;
    }
    if(a[a[0]+1])a[0]++;
    for (int i = a[0];i>=1;i--){
        printf("%d ",a[i]);
    }
    printf("\n");
}

int echo(){     //判断回文
    int yes = 1;
    for (int i = 1;i<=a[0]/2;i++){
        if (a[i]!=a[a[0]-i+1])  {
            yes = 0; 
            break;
        }
    }
    return yes;
}

int main(){
    scanf("%d",&n);
    while (getchar()!='\n') continue;
    char c;
    while ((c=getchar())!='\n'){
        for (int i = a[0];i>=1;i--){
           a[i+1]=a[i];
        }
        if (c>='0'&&c<='9'){
            a[1]=c-48;
        }
        if (c>='A'&&c<='Z'){
            a[1]=c-'A'+10;
        }
        a[0]++;
    }
    int step;
    for (step = 1;step<=30;step++){
        highplusecho();
        if(echo()) break;
    }
    if(step <= 30) printf("STEP=%d",step);
    else printf("Impossible!");
    return 0;
}```