LeetCode: 234. Palindrome Linked List

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234. Palindrome Linked List

LeetCode - Palindrome Linked List

使用快慢指针将链表切成两段，然后对后面的一段进行反转，再逐个比较两个子链。

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

Follow up:

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

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/**
 * Runtime: 1 ms, faster than 99.33% of Java online submissions for Palindrome Linked List.
 *
 * Memory Usage: 41.5 MB, less than 41.46% of Java online submissions for Palindrome Linked List.
 */
public boolean isPalindrome(ListNode head) {
    if (Objects.isNull(head) || Objects.isNull(head.next)) {
        return true;
    }

    ListNode slow = head;
    ListNode fast = head;
    while (Objects.nonNull(fast) && Objects.nonNull(fast.next)) {
        slow = slow.next;
        fast = fast.next.next;
    }
    slow = reverseList(slow);
    fast = head;
    while (Objects.nonNull(slow)) {
        if (slow.val != fast.val) {
            return false;
        }
        slow = slow.next;
        fast = fast.next;
    }
    return true;
}

private ListNode reverseList(ListNode head) {
    ListNode result = null;
    while (Objects.nonNull(head)) {
        ListNode next = head.next;
        head.next = result;
        result = head;
        head = next;
    }
    return result;
}