itoa函数

前两天写了下atol，今天下午就想了想写了itoa；没想到这个还挺复杂函数原型

char *itoa( int value, char *str);

char *itoa(int value, char *str)
{
    char dig[] = "0123456789";
    int i = 0, neg = 1;
    char *s, *e;
    char mid;
    long q = 0;
    int r = 0;

    if(str == NULL)
        return NULL;
    if(value > 0)
    {
        neg = 0;
        value = -value;
    }
    while(value <= -10)
    {
        q = value / 10;
        r = value % 10;
        if(r > 0)
        {
            r -= 10;
            q++;
        }
        str[i++] = (-r) + dig[0];
        value = q;
    }
    str[i] = (-value) + dig[0];
    if(neg == 1){
        str[i+1] = '-';
        str[i+2] = '\0';
        e = str + (i+1);
    }else{
        str[i+1] = '\0';
        e = str + i;
    }

    for( s = str; s < e; s++, e--)
    {
        mid = *s;
        *s = *e;
        *e = mid;
    }
    return str;
}