Determines if a linked list is a palindrome. Both solutions work for a singly or doubly linked list.

FindPalindromeLinkedList.swift

func isPalindrome(_ list: DoublyLinkedList<String>) -> Bool {
    // Simple solution. Iterate over linked list and store as string.
    // Uses linear space and time.
    // Then compare string to a reversed string.
    // If equal, then we have a palindrome.
    var node = list.first
    var string = ""
    while node != nil {
        string += String(node!.element)
        node = node!.next
    }
    return string == String(string.reversed())
}

func isPalindrome(_ list: SinglyLinkedList<String>) -> Bool {
    // Recursive solution. Uses constant space and linear time.
    // Checks first and last elements in linked list.
    // Removes elements from front and back until there
    // is one element, no elements, or an invalid palindrome is found.
    if list.isEmpty || list.first === list.last {
        return true
    }
    if list.first!.element == list.last!.element {
        _ = list.remove(node: list.first!)
        _ = list.remove(node: list.last!)
        return isPalindrome(list)
    }
    return false
}