//ignore header files
int atoi(char *);
void itoa(int,char *);
void reverse(char *);
int main(int argc, char const* argv[])
{
    char s[] = "12345";
    //string to integer
    int i = atoi(s);
    printf("%d\n",i);
    char *a = (char *)malloc(50);
    //integer to string
    itoa(i,a);
    //12345
    printf("%s\n",a);
    itoa(-i,a);
    //-12345
    printf("%s\n",a);
    return 0;
}
int atoi(char *s){
    int sum = 0;
    for(int i = 0;s[i] != '\0';i++){
        //注意要扣掉'0'
        sum = sum*10+s[i]-'0';
    }
    return sum;
};
void itoa(int n,char *s){
    int flag = 1;
    if (n<0){
        n = -n;
        flag = 0;
    }
    int i = 0;
    while(n != 0){
        //注意要把'0'加回來
        s[i++] = n%10+'0';
        n = n/10;
    }
    if(!flag)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
}
void reverse(char *s){
    for(int i = 0,j = strlen(s)-1;i<j;i++,j--){
        int c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
}