203. 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

比较简单的题目，但是也写错了很多次，因为考虑了很多p = head, q = head, 如果为tail的情况，但其实可以直接加一个前导node在head前面，这样简单很多。

/**
 * 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 NULL;
        ListNode *pre_head = new ListNode(0);
        pre_head->next = head;    //记得设置pre_head的next
        ListNode* pre = pre_head;
        ListNode* cur = head;
        while (cur) {
            if (cur->val == val) {//if else的逻辑要搞清楚
                pre->next = cur->next;
                cur = pre->next;
            }
            else {
                pre = cur;
                cur = cur->next;
            }
        }
        
        return pre_head->next;// 返回的是pre_head->next, 不是head
    }
};