19. Remove Nth Node From End of List

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Follow up: Could you do this in one pass?

Example 1:

Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]

Example 2:

Input: head = [1], n = 1
Output: []

Example 3:

Input: head = [1,2], n = 1
Output: [1]

Constraints:

• The number of nodes in the list is sz.
• 1 <= sz <= 30
• 0 <= Node.val <= 100
• 1 <= n <= sz

# @lc code=start
using LeetCode

# Double Pointers
function remove_nth_from_end_by_double_pointers!(
    head::ListNode, n::Int
)::Union{Nothing,ListNode}
    p1 = p2 = head
    for _ in 1:n
        p2 = next(p2)
    end
    # remove the first node
    isnothing(p2) && return next(head)
    p2 = p2.next ## let distance(p1, p2) = n + 1
    while !isnothing(p2)
        p1, p2 = next(p1), next(p2)
    end
    p1.next = p1.next.next ## remove one node
    return head
end
# @lc code=end

remove_nth_from_end_by_double_pointers! (generic function with 1 method)

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