82. Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate
numbers, leaving only distinct numbers from the original list.

https://leetcode.com/problems...

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def deleteDuplicates(self, head: ListNode) -> ListNode:
        # Two nodes: one to keep head, one to keep tail.
        ahead = phead = ListNode('null')

        # Two pointers: one to move ahead, one to monitor duplicates.
        p = None
        c = head

        # When move to end, terminate.
        while c:
            # At the beginning, simple move one step forward.
            if p == None:
                p = c
            # If p and c have different values, need to determine why.
            elif p.val != c.val:
                # If p and c are neighbors, p must be unique value.
                if p.next == c:
                    phead.next = p
                    phead = phead.next
                    phead.next = None
                    # p gets one step forward.
                    p = c
                # If p and c are not neighbors, ignore nodes with p.val.
                else:
                    p = c
            # c moves one step anyway.
            c = c.next

        # If p is not None (input not empty) and has next, collect last element.
        if p != None and p.next == None:
            phead.next = p

        return ahead.next