[LeetCode]#206. Reverse Linked List

Environment: Python 3.8

Key technique: List node

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]

Analysis:

1. Use below four steps to revise link.
2. temp=head
3. head=head.next
4. temp.next=ans
5. ans=temp

Solution:

class ListNode:
     def __init__(self, val=0, next=None):
         self.val = val
         self.next = next
head = [1,2,3]
head=ListNode(1)
head.next=ListNode(2)
head.next.next=ListNode(3)
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        ans = None
        while head:
            temp = head
            head = head.next
            temp.next = ans
            ans = temp
        return ans

Submissions:

Reference:

https://leetcode.com/problems/reverse-linked-list/discuss/1352989/Python-O(n)-time-O(1)-space