234. Palindrome Linked List

判断一个链表是不是回文链表

获取链表的中点，根据快慢指针，将后半部分反转，然后从中点逐个比较。

• Runtime: 187 ms, faster than 68.01%
• Memory Usage: 74.7 MB, less than 22.36%
• 时间复杂度O(n)，空间复杂度O(n)

/**
 * 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 half = getHalf(head);
  half.next = reverse(half.next);
  let p1 = head;
  let p2 = half.next;
  while(p2) {
    if(p1.val !== p2.val) {
      return false;
    }
    p1 = p1.next;
    p2 = p2.next;
  }
  return true;
};

var getHalf = function(node) {
  let slow = node;
  let fast = node;
  while (fast.next && fast.next.next) {
    slow = slow.next;
    fast = fast.next.next;
  }
  return slow;
}

var reverse = function(node) {
  let prev = null;
  let cur = node;
  while (cur) {
    let tmp = cur.next;
    cur.next = prev;
    prev = cur;
    cur = tmp;
  }
  return prev;
}