203. Remove Linked List Elements

展現一下dummy指針的用處。（開始操作時head可能需要上拉到dummy）

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        head = dummy;

        while(head->next){
            if(head->next->val==val){
                head->next = head->next->next;
            }
            else{
                head = head->next;
            }
        }

        return dummy->next;
    }


};