452. Remove Linked List Elements

 ListNode类

public class ListNode {
	int val;
	ListNode next;

	public ListNode(int val) {
		this.val = val;
		this.next = null;
	}
}

方法一：不使用虚拟头结点

public class Solution{
	/**
	 * @param head: a ListNode
	 * @param val: An integer
	 * @return: a ListNode
	 */
	public ListNode removeElements(ListNode head, int val) {
		while (head != null && head.val == val) {
			ListNode delNode = head;
			head = head.next;
			delNode.next = null;
		}
		if (head == null) {
			return null;
		}
		ListNode prev = head;
		while (prev.next != null) {
			if (prev.next.val == val) {
				ListNode delNode = prev.next;
				prev.next = delNode.next;
				delNode.next = null;
			} else {
				prev = prev.next;
			}
		}
		return head;
	}
}

方法二：使用虚拟头结点

public class Solution{
	/**
	 * @param head: a ListNode
	 * @param val: An integer
	 * @return: a ListNode
	 */
	public ListNode removeElements(ListNode head, int val) {
		ListNode dummyHead = new ListNode(-1);
		dummyHead.next = head;
		ListNode prev = dummyHead;
		while (prev.next != null) {
			if (prev.next.val == val) {
				ListNode delNode = prev.next;
				prev.next = delNode.next;
				delNode.next = null;
			} else {
				prev = prev.next;
			}
		}
		return dummyHead.next;
	}
}

方法三：使用递归

public class Solution {
	/**
	 * @param head: a ListNode
	 * @param val: An integer
	 * @return: a ListNode
	 */
	public ListNode removeElements(ListNode head, int val) {
		if (head == null) {
			return null;
		}
		ListNode ret = removeElements(head.next, val);
		if (head.val == val) {
			return ret;
		} else {
			head.next = ret;
			return head;
		}
	}
}

使用递归优化代码

public class Solution {
	/**
	 * @param head: a ListNode
	 * @param val: An integer
	 * @return: a ListNode
	 */
	public ListNode removeElements(ListNode head, int val) {
		if (head == null) {
			return null;
		}
		head.next = removeElements(head.next, val);
		return head.val == val ? head.next: head;
	}
}