LeetCode-234. Palindrome Linked List

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问题描述

Given the head of a singly linked list, return true if it is a palindrome.

Constraints:

  • The number of nodes in the list is in the range [1, 105].
  • 0 <= Node.val <= 9

Follow up: Could you do it in O(n) time and O(1) space?

解答

找到链表的后半部分，将后半部分反转，再跟前半部分比较

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/*
 * @Description: 234. Palindrome Linked List
 * @Author: libk
 * @Github: https://github.com/libk
 */
/**
 * 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}
 */
const isPalindrome = function (head) {
  if (head === null || head.next === null) {
    return true
  }
  let fast = head
  let slow = head
  while (fast !== null && fast.next !== null) {
    slow = slow.next
    fast = fast.next.next
  }

  // 如果fast不是null，说明链表节点的个数是奇数个
  if (fast !== null) {
    slow = slow.next
  }
  slow = reverseList(slow)
  fast = head
  while (slow !== null) {
    if (slow.val !== fast.val) {
      return false
    }
    slow = slow.next
    fast = fast.next
  }
  return true
}

function reverseList (head) {
  if (head === null || head.next === null) {
    return head
  }
  let pre = null
  let cur = head
  while (cur !== null) {
    let temp = cur.next
    cur.next = pre
    pre = cur
    cur = temp
  }
  return pre
}