recursion.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null) {
            return null;
        }
        if (head.next == null || head.val != head.next.val) {
            head.next = deleteDuplicates(head.next);
            return head;
        } else {
            ListNode ptr = head;
            while (ptr != null && ptr.val == head.val) {
                ptr = ptr.next;
            }
            return deleteDuplicates(ptr);
        }
    }
}