add util func to build linked list of vector from string data

This line triggers unused variable warning, so I deleted this line

On 12/20/14, metacpp <notifications@github.com> wrote:
>> @@ -40,7 +40,6 @@ ListNode* build_linked_list(const string& list_data) {
>>    // Use stream to split string with delimiter = ",".
>>    istringstream stream(list_data);
>>    string token = "";
>> -  char delimiter = ',';
>>
>>    // Build the linked list.
>>    while (getline(stream, token, ',')) {
>
> I guess that you're going to use delimiter here?
>
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> https://github.com/metacpp/LeetCpp/pull/1/files#r22141660