Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.

Example 1:

Input: head = [1,1,2]
Output: [1,2]
Example 2:

Input: head = [1,1,2,3,3]
Output: [1,2,3]

Constraints:

The number of nodes in the list is in the range [0, 300].
-100 <= Node.val <= 100
The list is guaranteed to be sorted in ascending order.

來源：力扣（LeetCode）
連結：https://leetcode.cn/problems/remove-duplicates-from-sorted-list

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

    /* apply the recursion technique*/
    public ListNode deleteDuplicates(ListNode head) {
        if(null == head || null == head.next){
            return head;
        }
        head.next = deleteDuplicates(head.next);
        return head.next.val == head.val ? head.next : head ;
    }
}