Cracking the Coding Interview Q2.7

public static boolean isPalindrome(LinkedListNode head) {
LinkedListNode fast = head;
LinkedListNode slow = head;

Stack<Integer> stack = new Stack<Integer>();

while (fast != null && fast.next != null) {
stack.push(slow.data);
slow = slow.next;
fast = fast.next.next;
}

/* Has odd number of elements, so skip the middle */
if (fast != null) {
slow = slow.next;
}

while (slow != null) {
int top = stack.pop().intValue();
System.out.println(slow.data + " " + top);
if (top != slow.data) {
return false;
}
slow = slow.next;
}
return true;
}

public Question.Result isPalindromeRecurse(LinkedListNode head, int length) {
if (head == null || length == 0) {
return new Question.Result(null, true);
} else if (length == 1) {
return new Question.Result(head.next, true);
} else if (length == 2) {
return new Question.Result(head.next.next, head.data == head.next.data);
}
Question.Result res = isPalindromeRecurse(head.next, length - 2);
if (!res.result || res.node == null) {
return res; // Only "result" member is actually used in the call stack.
} else {
res.result = head.data == res.node.data;
res.node = res.node.next;
return res;
}
}

public boolean isPalindrome(LinkedListNode head) {
int size = 0;
LinkedListNode n = head;
while (n != null) {
size++;
n = n.next;
}
Result p = isPalindromeRecurse(head, size);
return p.result;
}