Palindrome Linked List

The problem. Given the head of a singly linked list, return true if it is a palindrome — it reads the same forwards and backwards — and false otherwise.

1 -> 2 -> 2 -> 1 is a palindrome: the outer pair 1 and 1 match, and the inner pair 2 and 2 match. 1 -> 2 is not.

Example

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

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

With an array you'd compare arr[i] with arr[n - 1 - i]. A singly linked list can only walk forwards, so reaching "the second-to-last node" is the hard part.

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Still to come

1. Brute force: walk to each mirror node from the head
2. Optimal: copy the values into an array
3. Best solution: reverse the second half in place
Comparing the three approaches
Edge cases to test

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Palindrome Linked List

Example

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