56-Remove Linked List Elements

1. Remove Linked List Elements My Submissions QuestionEditorial Solution
   Total Accepted: 61924 Total Submissions: 215788 Difficulty: Easy
   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
思路：先找到第一个不等于指定值的首节点，作为返回值
另外在过程中过滤掉等于val值的节点并使前面一个不为val值得节点指向下一个不为val值的节点
时间复杂度：O(n)
空间复杂度：O(1)

/**
 * 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;
        while(head!=NULL&&head->val==val)head=head->next;//找到第一个头部
        ListNode *p=head,*prep=head;
        while(p!=NULL){
            prep=p;
            p=p->next;
            while(p!=NULL&&p->val==val)p=p->next;//直到找到下一个不为val的节点并指向它
            prep->next = p;
        }
        return head;
    }
};