Palindrome Linked List in O(n) Time and O(1) Space using JavaScript

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:

The number of nodes in the list is in the range [1, 10⁵].
0 <= Node.val <= 9

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

Understanding the Problem

The core challenge of this problem is to determine if the values in a singly linked list form a palindrome. A palindrome is a sequence that reads the same backward as forward. This problem is significant in various applications such as text processing, data validation, and more.

Potential pitfalls include handling edge cases like an empty list or a list with a single node, both of which are trivially palindromes.

Approach

To solve this problem, we can consider the following approaches:

Naive Approach

The naive approach involves copying the values of the linked list into an array and then checking if the array is a palindrome. This approach is simple but requires O(n) space, which is not optimal.

Optimized Approach

To achieve O(n) time and O(1) space complexity, we can use the following steps:

Find the middle of the linked list using the slow and fast pointer technique.
Reverse the second half of the linked list.
Compare the first half and the reversed second half of the list.
Restore the original list structure (optional).

Algorithm

Let's break down the optimized algorithm step-by-step:

Find the middle of the linked list: Use two pointers, slow and fast. Move slow by one step and fast by two steps. When fast reaches the end, slow will be at the middle.
Reverse the second half: Reverse the nodes starting from the middle to the end of the list.
Compare the two halves: Compare the values of the nodes in the first half and the reversed second half.
Restore the list (optional): Reverse the second half again to restore the original list structure.

Code Implementation

/**
 * 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 || !head.next) return true;

    // Step 1: Find the middle of the linked list
    let slow = head, fast = head;
    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }

    // Step 2: Reverse the second half of the list
    let prev = null, curr = slow;
    while (curr) {
        let nextTemp = curr.next;
        curr.next = prev;
        prev = curr;
        curr = nextTemp;
    }

    // Step 3: Compare the first half and the reversed second half
    let firstHalf = head, secondHalf = prev;
    while (secondHalf) {
        if (firstHalf.val !== secondHalf.val) return false;
        firstHalf = firstHalf.next;
        secondHalf = secondHalf.next;
    }

    // Optional Step 4: Restore the list (not required for the problem)
    // Reverse the second half again to restore the original list
    // prev = null, curr = slow;
    // while (curr) {
    //     let nextTemp = curr.next;
    //     curr.next = prev;
    //     prev = curr;
    //     curr = nextTemp;
    // }

    return true;
};

Complexity Analysis

The time complexity of this approach is O(n) because we traverse the list a constant number of times. The space complexity is O(1) because we only use a few pointers and do not allocate extra space proportional to the input size.

Edge Cases

Consider the following edge cases:

Empty list: Should return true.
Single node list: Should return true.
List with all identical elements: Should return true.
List with different elements: Should return false.

Testing

To test the solution comprehensively, consider the following test cases:

head = [1, 2, 2, 1] // true
head = [1, 2] // false
head = [1] // true
head = [] // true
head = [1, 2, 3, 2, 1] // true
head = [1, 2, 3, 4, 5] // false

Use a testing framework like Jest or Mocha to automate the testing process.

Thinking and Problem-Solving Tips

When approaching such problems, consider the following tips:

Break down the problem into smaller, manageable parts.
Think about different approaches and their trade-offs.
Practice solving similar problems to improve your problem-solving skills.

Conclusion

In this blog post, we discussed how to determine if a singly linked list is a palindrome using an optimized approach with O(n) time and O(1) space complexity. We covered the problem definition, approach, algorithm, code implementation, complexity analysis, edge cases, and testing. Understanding and solving such problems is crucial for improving your algorithmic thinking and coding skills.

Additional Resources

For further reading and practice, consider the following resources: