删除单链表中指定的val的所有节点。
class Solution { public: ListNode* removeElements(ListNode* head, int val) { if (NULL == head) { return NULL; } ListNode * before = NULL; ListNode * now = head; while (now != NULL) { if (now -> val == val) { if (before == NULL) { head = head->next; now = head; } else { if (now != NULL) { before->next = now->next; } now = before->next; } } else { before = now; now = now->next; } } return head; } };