234. 回文链表

方法一：转成数组

/**
 * 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 {boolean}
 */
var isPalindrome = function (head) {
  let valArr = []
  while (head) {
    valArr.push(head.val)
    head = head.next
  }
  for (let i = 0, j = valArr.length - 1; i < j; i++, j--) {
    if (valArr[i] !== valArr[j]) {
      return false
    }
  }
  return true
}

方法二：快慢指针

/**
 * 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 {boolean}
 */
var isPalindrome = function (head) {
  if (head == null || head.next == null) {
    return true
  }
  let fast = head
  let slow = head
  let prev
  while (fast && fast.next) {
    prev = slow
    slow = slow.next
    fast = fast.next.next
  }
  prev.next = null // 断成两个链表
  // 翻转后半段
  let head2 = null
  while (slow) {
    const tmp = slow.next
    slow.next = head2
    head2 = slow
    slow = tmp
  }
  // 比对
  while (head && head2) {
    if (head.val != head2.val) {
      return false
    }
    head = head.next
    head2 = head2.next
  }
  return true
}