Leetcode 206 Reverse Linked List

206-Reverse-Linked-List.py

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

# 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 is None:
            return None

        if head.next is None:
            return head

        a = head
        b = a.next
        a.next = None

        while b is not None:
            # i = 0
            # 1 -> 2 -> 3 -> 4 -> 5
            # a    b    c
            # 1 <- 2,   3 -> 4 -> 5
            #      a    b
            #
            # i = 1
            # 1 <- 2,   3 -> 4 -> 5
            #      a    b    c
            # 1 <- 2 <- 3,   4 -> 5
            #           a    b
            #
            # i = 2
            # 1 <- 2 <- 3,   4 -> 5
            #           a    b    c
            # 1 <- 2 <- 3 <- 4    5
            #                a    b
            #
            # i = 3
            # 1 <- 2 <- 3 <- 4,   5
            #                a    b
            # 1 <- 2 <- 3 <- 4 <- 5
            #                     a
            c = b.next
            b.next = a
            a = b
            b = c

        return a