LeetCode/206_Reverse_Linked_List.py at main · AnmolPatel20/LeetCode · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
"""
Given the head of a singly linked list, reverse the list, and return the reversed list.


Example 1:


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


Input: head = [1,2]
Output: [2,1]
Example 3:

Input: head = []
Output: []


Constraints:

The number of nodes in the list is the range [0, 5000].
-5000 <= Node.val <= 5000


Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

"""

# 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]:
        prev = None
        temp = head
        while temp:
            nxt = temp.next
            temp.next = prev
            prev = temp
            temp = nxt
        return prev