# 18.\[Linked list]:234. Palindrome Linked List

23\. Merge k Sorted ListsHard17.3K624Companies

You are given an array of `k` linked-lists `lists`, each linked-list is sorted in ascending order.

*Merge all the linked-lists into one sorted linked-list and return it.*

**Example 1:**

<pre><code><strong>Input: lists = [[1,4,5],[1,3,4],[2,6]]
</strong><strong>Output: [1,1,2,3,4,4,5,6]
</strong><strong>Explanation: The linked-lists are:
</strong>[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6
</code></pre>

**Example 2:**

<pre><code><strong>Input: lists = []
</strong><strong>Output: []
</strong></code></pre>

**Example 3:**

<pre><code><strong>Input: lists = [[]]
</strong><strong>Output: []
</strong></code></pre>

**Constraints:**

* `k == lists.length`
* `0 <= k <= 104`
* `0 <= lists[i].length <= 500`
* `-104 <= lists[i][j] <= 104`
* `lists[i]` is sorted in **ascending order**.
* The sum of `lists[i].length` will not exceed `104`.

{% embed url="<https://leetcode.com/problems/palindrome-linked-list/>" %}

### With array

> time: O(N)

> space: O(N)

```jsx
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {boolean}
 */
var isPalindrome = function(head) {
    if (head === null) return true;
    if (head.next === null) return true;

    const arr = [];
    let curr = head;
    while (curr !== null) {
        arr.push(curr.val);
        curr = curr.next;
    }

    let left = 0;
    let right = arr.length - 1;
    while (left <= right) {
        if (arr[left] !== arr[right]) return false;
        left++;
        right--;
    }

    return true;
};
```

### With reversing array

> time: O(N)

> space: O(1)

```jsx

var isPalindrome = function(head) {
    if (head === null) return true;
    if (head.next === null) return true;

    let one = head;
    let two = head;
    while (two.next !== null && two.next.next !== null) {
        one = one.next;
        two = two.next.next;
    }

    // 1 2 3 => one is 1 => pivot should be 3
    // 1 2 3 4 => one is 2 => pivot should be 3
    // so one.next is pivot for odd and even
    const pivot = one.next;

    one = head;
    two = reverse(pivot);
    while (two !== null) {
        if (two.val !== one.val) return false;
        one = one.next;
        two = two.next;
    }

    return true;
};

function reverse(node) {
    let prev = null;
    let curr = node;
    while (curr !== null) {
        const next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }

    return prev;
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://algorithm.prettylog.com/algorithm-problems/daily-algorithms/18.-linked-list-234.-palindrome-linked-list.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.