---
title: 'LeetCode 234. Palindrome Linked List'
disqus: hackmd
---

# LeetCode 234. Palindrome Linked List

## Description

Given the head of a singly linked list, return true if it is a palindrome.

## Example

Input: head = [1,2,2,1]
Output: true

## Constraints

The number of nodes in the list is in the range [1, 10^5^].
0 <= Node.val <= 9

## Answer

此題可先用slow & fast抓中間點，然後從中間做反轉，最後一一比對值有無相同。

bool isPalindrome(struct ListNode* head){
    if(head == NULL){return false;}
    if(head->next == NULL){return true;}
    struct ListNode *slow = head->next, *fast = head->next->next;
    struct ListNode *cur = NULL, *nxt = NULL, *opr = NULL;
    while(fast != NULL && fast->next != NULL){
        slow = slow->next;
        fast = fast->next->next;
    }
    opr = slow;
    while(opr != NULL){
        nxt = opr->next;
        opr->next = cur;
        cur = opr;
        opr = nxt;
    }
    while(cur != NULL){
        if(cur->val == head->val){
            cur = cur->next;
            head = head->next;
        }
        else{
            return false;
        }
    }
    return true;
}

## Link
https://leetcode.com/problems/palindrome-linked-list/
###### tags: `Leetcode`