19. Remove Nth Node From End of List

# 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, n: int):
        p1 = head
        p2 = head

        for _ in range(n):
            p2 = p2.next  # fast

        if not p2:
            return head.next # in case: head=[1], n=1 -> return []

        while p2.next:
            p1 = p1.next
            p2 = p2.next

        p1.next = p1.next.next

        return head

# 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: Optional[ListNode], n: int) -> Optional[ListNode]:
        if not head.next:
            return None

        nodes = {}
        cur = head

        i = 0
        while cur:
            nodes[i] = cur
            cur = cur.next
            i += 1
        
        if i - n == 0:
            return head.next

        cur = nodes[i - n - 1]

        if cur.next:
            cur.next = cur.next.next
        return head