動機

複習linked list的reverse

Problem

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?

Sol

寫iterative版的reverse感覺很像在寫rotate image之類的題目，都要把不同區段用變數列出來，之後在loop中保持他們的狀態

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if head is None:
            return None
        elif head.next is None:
            return head
        else:
            now = head.next
            prev = head
            nextNow = head.next

            last = prev
            while now:
                nextNow = nextNow.next
                now.next = prev
                prev = now
                now = nextNow
            last.next = None
                
            return prev