LeetCode 206. Reverse Linked List

Question

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

206ex1 
Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

 

Example 2:

206ex2 
Input: head = [1,2]
Output: [2,1]

 

Example 3:

Input: head = []
Output: []

 

Constraints:

  • 2 <= nums.length <= 500
  • 0 <= nums[i] <= 100

 

 

Solution

Python

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if head == None:
            return head
        else:
            p = head
            while p.next:
                n = p.next
                p.next = n.next
                n.next = head
                head = n
            return head

Java