234. Palindrome Linked List
题目:
Given a singly linked list, determine if it is a palindrome.
Follow up:
Could you do it in O(n) time and O(1) space?
链接: http://leetcode.com/problems/palindrome-linked-list/
2/28/2017
参考别人的答案,因为一开始不知道可不可以改变list的结构。
1 public class Solution { 2 public boolean isPalindrome(ListNode head) { 3 if (head == null || head.next == null) return true; 4 ListNode mid = findMidNode(head); 5 ListNode ret = reverseList(mid); 6 7 while(ret != null) { 8 if (head.val != ret.val) return false; 9 head = head.next; 10 ret = ret.next; 11 } 12 return true; 13 } 14 private ListNode findMidNode(ListNode head) { 15 ListNode slow = head; 16 ListNode fast = head; 17 18 while (fast != null) { 19 fast = fast.next; 20 if (fast != null) { 21 fast = fast.next; 22 slow = slow.next; 23 } 24 } 25 return slow; 26 } 27 private ListNode reverseList(ListNode head) { 28 ListNode dummy = new ListNode(0); 29 ListNode tmp; 30 while (head != null) { 31 tmp = head.next; 32 head.next = dummy.next; 33 dummy.next = head; 34 head = tmp; 35 } 36 return dummy.next; 37 } 38 }