Flip the Chain

Easy

Train cars are coupled into a singly linked list: each node knows only the next one. You are given the list's head. Recouple the cars so they run in the opposite order, and return the new head, which is the former last node. An empty list (head is None) stays empty. Try solving it two ways: with a loop and with recursion.

Example 1

head=list_node([7, -2, 0, 15]) → [15, 0, -2, 7]

The nodes come in reverse order: the last one (15) becomes the head and the first one (7) becomes the tail.

Example 2

head=list_node([42]) → [42]

A single-node list stays the same when reversed.

Example 3

head=list_node([]) → None

An empty list stays empty.

Constraints

0 ≤ number of nodes ≤ 5000
-5000 ≤ Node.val ≤ 5000

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Topics

Recursion Linked List

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Input

head=list_node([7, -2, 0, 15])

Expected

[15, 0, -2, 7]

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