203. Remove Linked List Elements

题目描述

Remove all elements from a linked list of integers that have value val.

Example:

Input: 1->2->6->3->4->5->6, val = 6
Output: 1->2->3->4->5

方法思路

Approach1:

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        while (pre.next != null) {
            if (pre.next.val == val) pre.next = pre.next.next;
            else pre = pre.next;
        }
        return dummy.next;
        //为什么不可以直接 return head;还是想不明白
    }
}

Approach2:递归

class Solution{
// 3 line recursively solution   Runtime: 4 ms, faster than 77.84%
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return null;
        }
        head.next = removeElements(head.next, val);
        if(head.val == val)
            return head.next;
        return head;//递归的方法
}
}