leetcode 234 Palindrome Linked List
z
 2017-12-26 15:58:36  

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?

  • using two pointers:
    • A slow pointer with step length equals One
    • A fast pointer with step length eauqls Two
  • We can get the middle elements pointed by slow when the total number of the array is odd.
  • Slow pointer points at the larger one when the array is at a length of even.
  • We can implements an function which reverses the linked list, So we can get the reverse list over [slow, -1].
  • compare head with slow step by step until slow points to null.
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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }
        slow = reverse(slow);
        while(slow!=null){
            if(slow.val != head.val){
                return false;
            }
            head = head.next;
            slow = slow.next;
        }
        return true;
    }
    public ListNode reverse(ListNode head){
        ListNode prev = null;
        while(head != null){
            ListNode next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}