203. Remove Linked List Elements

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

本题比较简单，构造一个dummy头即可。

移位删除时，存在与不存在的删除办法不同

/**
 * 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 prev = dummy;
        ListNode curr = dummy.next;
        while(curr.next != null) {
            if(curr.val == val) {
                prev.next = curr.next;
                curr = curr.next;
            }
            else {
                prev = prev.next;
                curr = curr.next;
            }
        }
        if(curr.next == null) {
            if(curr.val == val) prev.next = null;
        }
        return dummy.next;
        
    }
}