Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->2->3.

Iterative Solution, O(n) time, O(1) space

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public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode current = head;
        while (current != null && current.next != null){
            if (current.val == current.next.val){
                current.next = current.next.next;
            }else{
                current = current.next;
            }
        }
        return head;
    }
}

Recursive Solution, O(n) time, O(n) space

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public 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;
    }
}