Palindrome linked list

Check whether a list reads the same forwards and backwards by reversing its second half and comparing inward from both ends.

The idea

Find the middle with slow and fast pointers, reverse the list from the middle to the end in place, then walk a pointer in from each end comparing values. If every pair matches before the reversed half runs out, the list is a palindrome.

Code

public static bool IsPalindrome(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) {
        slow = slow.next; fast = fast.next.next;
    }
    ListNode second = Reverse(slow);
    ListNode p1 = head, p2 = second;
    while (p2 != null) {
        if (p1.val != p2.val) return false;
        p1 = p1.next; p2 = p2.next;
    }
    return true;
}

Tips

• This reuses two earlier patterns back to back: the fast/slow middle finder, then the in-place reversal.
• Stop and return false the moment a pair disagrees — there is no need to keep comparing.

In the .NET library

This check mutates the list (the second half ends up reversed) while it runs. Production code usually reverses it back afterwards so the caller sees the original order.