206. Reverse Linked List

https://leetcode.com/problems/reverse-linked-list

Python

Recursion

class Solution:
   def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
       return self._travel(head, None)

   def _travel(self, node, prev):
       if not node:
           return prev

       origin_next = node.next
       node.next = prev

       return self._travel(origin_next, node)

Loop

(This is be used in 143. Reorder List)

class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head:
            return

        prev = None
        while head:
            head.next, prev, head =  prev, head, head.next

        return prev

Go

Recursion

func reverseList(head *ListNode) *ListNode {
    return travel(head, nil)
}

func travel(node *ListNode, prev *ListNode) *ListNode {
    if node == nil {
        return prev
    }

    originNext := node.Next
    node.Next = prev

   return travel(originNext, node)
}