203. Remove Linked List Elements

展现一下dummy指针的用处。（开始操作时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;    }};