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Given the head
of a singly linked list, return true
if it is a palindrome.
Example 1:
Input: head = [1,2,2,1] Output: true
Example 2:
Input: head = [1,2] Output: false
Constraints:
[1, 105]
.0 <= Node.val <= 9
Follow up: Could you do it in
O(n)
time and O(1)
space?
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def isPalindrome(self, head: ListNode) -> bool:
if head is None or head.next is None:
return True
slow, fast = head, head.next
while fast and fast.next:
slow, fast = slow.next, fast.next.next
pre, cur = None, slow.next
while cur:
t = cur.next
cur.next = pre
pre, cur = cur, t
while pre:
if pre.val != head.val:
return False
pre, head = pre.next, head.next
return True
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isPalindrome(ListNode head) {
if (head == null || head.next == null) {
return true;
}
ListNode slow = head;
ListNode fast = head.next;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
ListNode cur = slow.next;
slow.next = null;
ListNode pre = null;
while (cur != null) {
ListNode t = cur.next;
cur.next = pre;
pre = cur;
cur = t;
}
while (pre != null) {
if (pre.val != head.val) {
return false;
}
pre = pre.next;
head = head.next;
}
return true;
}
}
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