203. 移除链表元素

给你一个链表的头节点 head 和一个整数 val ，请你删除链表中所有满足 Node.val == val 的节点，并返回 新的头节点 。

输入：head = [1,2,6,3,4,5,6], val = 6
输出：[1,2,3,4,5]

输入：head = [7,7,7,7], val = 7
输出：[]

题解：

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} val
 * @return {ListNode}
 */
var removeElements = function (head, val) {
  let dummyHead = new ListNode();
  dummyHead.next = head;
  let prev = dummyHead;
  while (prev.next != null) {
    if (prev.next.val === val) {
      prev.next = prev.next.next;
    } else {
      prev = prev.next;
    }
  }
  return dummyHead.next;
};

206. 反转链表

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

输入：head = [1,2]
输出：[2,1]

输入：head = []
输出：[]

题解：

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function (head) {
  let prev = null;
  let current = head;
  while (current != null) {
    let nextTemp = new ListNode(null);
    nextTemp = current.next;
    current.next = prev;
    prev = current;
    current = nextTemp;
  }

  return prev;
};