leetcode-day6:Remove Linked List Elements(移除链表元素)

203. Remove linked List Elements
链表：听说用虚拟头节点会方便很多？

Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.

Example 1:

Input: head = [1,2,6,3,4,5,6], val = 6
Output: [1,2,3,4,5]

Example 2:

Input: head = [], val = 1
Output: []

Example 3:

Input: head = [7,7,7,7], val = 7
Output: []

Constraints:

The number of nodes in the list is in the range [0, 104].
1 <= Node.val <= 50
0 <= val <= 50

JAVA实现代码：

 
//未添加虚节点版本
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head==null){
            return head;
        }

        ListNode currentNode=head;
        ListNode nextNode=head.next;

        while(nextNode!=null){
            if(nextNode.val==val){
                currentNode.next=nextNode.next;
            }else{
                currentNode=nextNode;
            }
            nextNode=nextNode.next;
        }
        
        //删除头结点
        if(head.val==val){
            head=head.next;
        }
        return head;
    }
}

//添加虚节点版本
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head==null){
            return head;
        }

        ListNode dummy  = new ListNode(-1,head);
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur!=null){
            if(cur.val==val){
                pre.next=cur.next;
            }else{
                pre=cur;
            }
            cur=cur.next;
            
        }
        head=dummy.next;
        return head;
    }
}