Remove all elements from a linked list of integers that have value val.

Example

Given: 1 --> 2 --> 6 --> 3 --> 4 --> 5 --> 6, val = 6

Return: 1 --> 2 --> 3 --> 4 --> 5

Credits:

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利用快慢两个指针

注意设置的头结点是临时局部变量

ListNode* removeElements(ListNode* head, int val) {    ListNode dummy(0);    dummy.next = head;    ListNode* slow = &dummy, *fast = head;    while (fast != nullptr)    {        if (fast->val != val)        {            slow->next = fast;            slow = slow->next;        }        fast = fast->next;    }    slow->next = nullptr;    return dummy.next;}