83. Remove Duplicates from Sorted List [Easy]

/**
 * Runtime: 1 ms, faster than 28.08%
 * Memory Usage: 41.2 MB, less than 5.44%
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode pre = head, curr = head.next;
        while (curr != null) {
            if (curr.val != pre.val) {
                pre.next = curr;
                pre = curr;
            }
            curr = curr.next;
        }
        pre.next = null;
        return head;
    }
}

/**
 * Runtime: 2 ms, faster than 28.08%
 * Memory Usage: 41 MB, less than 18.23%
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        head.next = deleteDuplicates(head.next);
        return head.val == head.next.val ? head.next : head;
    }
}