1. Palindrome Linked List

class Solution {
    public boolean isPalindrome(ListNode head) {
        if(head == null || head.next == null) return true;
        List<Integer> l = new ArrayList<>();
        while(head != null){
            l.add(head.val);
            head = head.next;
        }
        int start = 0, end = l.size()-1;
        while(start < end){
            if(l.get(start) != l.get(end)) return false;
            start++;end--;
        }
        return true;
    }
}

class Solution {
    public boolean isPalindrome(ListNode head) {
        //Find Mid 
        ListNode fast = head;
        ListNode slow = head;
        
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        // 1, ||| 2.  ||||  3 --> slow = 2, fast = 3
        if(fast != null) slow = slow.next;
        
        //Reverse the list start from slow. 
        ListNode revHead = reverse(slow);
        
        while(revHead != null) {
            if(revHead.val != head.val) return false;
            else {
                revHead = revHead.next;
                head = head.next;
            }
        }
        return true;
    }
    
    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        while(head != null) {
            ListNode next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}