string to integer (implement atoi)
You seem to have too many checks for '\0'. They don't add anything. Also too many brackets, but better too many than too few.

Also, is long long guaranteed to be bigger than int? I don't think it is. Here's another version that doesn't need a bigger representation than that of int: 

int atoi(const char *str)
{
    int n = 0;
    int sign = 1;

    while (isspace(*str)) {
        ++str;
    }
    if (*str == '-') {
        sign = -1;
        ++str;
    } else if (*str == '+') {
        sign = 1;
        ++str;
    }
    while (isdigit(*str)) {
        if (n &gt; INT_MAX/10) { /* EDIT: protect against overflow */
            break;
        }
        n *= 10;
        int ch = *str - '0';

        if (n &gt; INT_MAX - ch) {
            break;
        }
        n += ch;
        ++str;
    }
    if (isdigit(*str)) {
        return sign == 1 ? INT_MAX : INT_MIN;
    }
    return sign * n;
}


EDIT: I added a check to prevent overflow of the n *= 10

EDIT2: improved to avoid using an unsigned int - it was a hack