Remove Linked List Elements

Problem

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

Solution

Concise Recursion: O(n) time, O(n) space

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;
    }
}

Iterative Solution: O(n) time, O(1) space

public class Solution {
    public ListNode removeElements(ListNode head, int val) {
        while (head != null && head.val == val) head = head.next;
        ListNode cur = head, prev = new ListNode(0); //cannot set prev == null, NullPointerException 
        while (cur != null) {
            if (cur.val == val) prev.next = null; //This is for corner case like one node linked list 
            else {
                prev.next = cur;
                prev = prev.next;
            }
            cur = cur.next;
        }
        return head;
    }
}

Analysis

This should be a typical problem for Linked List
The recursive solution uses head.next to remove the elements
The iterative solution uses two pointers prev and cur to literally check elements and move