Question

Implement a function to check if a linked list is a palindrome.

Solution

There are multiple solutions for this question.

First, maybe the simplest solution of all, is to compare the list with the reversed list (compare first half would be enough). This is a very nice idea.

Second solution is iterative approach to use a Stack. Alternatively, we can also use fast/slow pointer to find the mid point.

Third solution is recursive. We basically uses a public pointer to:

1. get starting value
2. check middle parts
3. get ending value
4. if starting == ending and middle part is valid, then true.

This code is pretty hard to think. Read it below.

Code

Recursive

private static LinkedListNode p;

public static boolean isPalindromeRecursive(LinkedListNode head) {
    p = head;
    int len = getListLength(head);
    return recursiveHelper(0, len - 1);
}

private static int getListLength(LinkedListNode node) {
    int count = 0;
    while (node != null) {
        count++;
        node = node.next;
    }
    return count;
}

public static boolean recursiveHelper(int from, int to) {
    if (from > to) {
        return true;
    } else if (from == to) {
        p = p.next;
        return true;
    } else {
        // first get fromVal, then check middle part, last, get toVal
        int fromVal = p.data;
        p = p.next;
        if (!recursiveHelper(from + 1, to - 1)) {
            return false;
        }
        int toVal = p.data;
        p = p.next;
        if (fromVal != toVal) {
            return false;
        }
    }
    return true;
}