Remove Linked List Elements

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

先处理链表为空/只有一个元素的情况, 其余的情况直接遍历链表即可.
需要注意的是head的移动问题.

/**
* 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) {
if(head == NULL) {
return head;
}
if(head->val == val && head->next == NULL) {
return NULL;
}

ListNode* pre = NULL;
ListNode* p = head;

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

return head;
}
};