LeetCode-203-链表-移除所有的指定元素

方案一：正常逻辑，不使用虚拟头节点或递归

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        
        while(head != null && head.val==val){
            ListNode delHead=head;
            head=head.next;
            delHead=null;
        }
        
        if(head == null)
            return null;
        
        ListNode prev=head;
        while(prev.next!=null){
            if(prev.next.val==val){
                prev.next=prev.next.next;
            }else{
                prev=prev.next;
            }     
        }
        
        return head;
        
    }
}

方案二：使用虚拟头节点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        
        //定义虚拟头节点
        ListNode dummyHead=new ListNode(0);
        dummyHead.next=head;
        
        ListNode prev = dummyHead;
        
        while(prev.next!=null){
            if(prev.next.val==val){
                prev.next=prev.next.next;
            }else{
                prev=prev.next;
            }     
        }
        
        return dummyHead.next;
        
    }
}

方案三：递归

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        
        if(head==null)
            return null;
        
        ListNode res = removeElements(head.next,val);
        
        if(head.val==val){
            return res;
        }else{
            head.next=res;
            return head;
        }
        
    }
}