还不够简洁明了？

    int atoi( const char *str ) {
        int answer = 0, symbol = 1, i = 0, n = 0;

        if( str == NULL ) {
            return 0;
        }
        for( i = 0; str[i] != '\0' && str[i] == ' '; ++i ) {} // 跳过前导空格.
        if( str[i] == '-' || str[i] == '+' ) { // 处理首个有效字符是符号位.
            symbol = str[i++] != '-' ? 1 : -1;
        }
        for( ; str[i] != '\0' && isdigit( str[i] ); ) {
            n = (str[i++] - '0') * symbol;
            if( (answer > INT_MAX / 10) || (answer == INT_MAX / 10 && n > INT_MAX % 10) ) { // 上溢出.
                return INT_MAX;
            }
            if( (answer < INT_MIN / 10) || (answer == INT_MIN / 10 && n < INT_MIN % 10) ) { // 下溢出.
                return INT_MIN;
            }
            answer = answer * 10 + n;
        }

        return answer;
    }