203 - Remove Linked List Elements

#easy
Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.

Example 1:

Input: head = [1,2,6,3,4,5,6], val = 6
Output: [1,2,3,4,5]

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        // head
        while ( head != NULL && head -> val == val ) {
            if ( head -> next != NULL )  head = head -> next;
            else return NULL;
        }

        ListNode * it = head;
        while ( it != NULL ) {

            // middle and end; be careful for continued duplicate.
            if ( it -> next != NULL && it -> next -> val == val ) {
                if ( it -> next -> next != NULL )  {
                    it -> next  = it -> next -> next;
                }
                else it -> next = NULL; 
            }
            else {
                it = it -> next;
            }

        }

        return head;
    }
};