19. Merge Intervals

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        if (not head) or (not head.next):
            return None
        
        pointer_left = head
        pointer_right = head
        
        # shift pointer right by n 
        for i in range(0, n):
            pointer_right = pointer_right.next
        # short case
        if not pointer_right:
            return head.next
        
        while pointer_right.next:
            pointer_left = pointer_left.next
            pointer_right = pointer_right.next
        
        # remove nth element
        pointer_left.next = pointer_left.next.next
        
        return head