while(columnNumber) {            
        num[ retSize++ ] = columnNumber % 26;
        columnNumber /= 26;
    }

char * convertToTitle(int columnNumber){
    char *s = malloc(sizeof(char) * 256);   //申请返回空间
    int count = 0;
    while(columnNumber){            //转数组
        if(columnNumber % 26 != 0)
            s[count++] = columnNumber % 26 +'A' - 1;
        else {                      //等于0的时候借位
            s[count++] = 'Z';
            columnNumber --;
        }
        columnNumber /= 26;
    }
    s[count] = 0;               //加入结束符
    for(int i = 0;i < count/2;i++){        //反转字符串
        s[i] = s[i] ^ s[count - i - 1];
        s[count - 1 - i] = s[i] ^ s[count - 1 - i];
        s[i] = s[i] ^ s[count - i - 1];
    }
    return s;
}

int titleToNumber(char * columnTitle){
    int count = 0;                      //统计数字
    for(int i = 0;columnTitle[i];++i){
        count *= 26;
        count += columnTitle[i] - 'A' + 1;  //1的对应补偿
    }
    return count;
}

char * smallestGoodBase(char * n){
    long long nval = 0,k;
    for(int i = 0;n[i];++i){        //字符串转整形
        nval *= 10;
        nval += n[i] -'0';
    }
    int mmax = log(nval)/log(2);    //m位数的最大值向下取整
    for(;mmax > 1;mmax--){             //枚举m尝试找到k  
        k = pow(nval,1.0/mmax);         //k一定等于向下取整   m=1的时候不能这么计算
        long long temp = nval;
        while(temp){
            if(temp % k != 1)   break;
            temp /= k;
        }
        if(temp == 0) break;
    }
    char *ans = malloc(sizeof(char) * (mmax + 1));//创建返回函数
    int i = 0;
    if(mmax == 1)  k = nval - 1;        //如果mmax = 1 则k等于最大值-1
    while(k){
        ans[i++] = k % 10 + '0';
        k /= 10;
    }
    ans[i] = 0;             //插入结束符
    for(int j = 0;j < i / 2;j++){       //反转字符串
        ans[j] = ans[j] ^ ans[i - 1 - j];
        ans[i - 1 - j] = ans[j] ^ ans[i - 1 -j];
        ans[j] = ans[j] ^ ans[i - 1 - j];
    }
    return ans;
}