LeetCode 203. Remove Linked List Elements

原题链接：https://leetcode.com/problems/remove-linked-list-elements/

题目：

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

题解：

建一个dunmy，dunmy.next = head，为了防止head.val 等于要被去掉的value时删掉head不方便表示。

Time Complexity: O(n), n是list长度. Space: O(1).

AC Java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return head;
        }
        
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        while(cur.next != null){
            if(cur.next.val == val){
                cur.next = cur.next.next;
            }else{
                cur = cur.next;
            }
        }
        return dummy.next;
    }
}

Recursion Method, stop condition 是 head == null, return head. 

Time Complexity: O(n), n 是list长度. Space: O(n), stack space.

AC Java:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return head;
        }
        head.next = removeElements(head.next, val);
        return head.val == val ? head.next : head;
    }
}

类似Delete Node in a Linked List.