"""

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.


"""

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        # if the head is None
        if head == None: return None
        prev = head
        
        # keep tracking of prev.next
        while prev.next != None:
            if prev.val == prev.next.val:
                prev.next = prev.next.next
            else:
                prev = prev.next
        return head
        
        # Any comments?
