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68 lines
1.6 KiB
Plaintext
68 lines
1.6 KiB
Plaintext
---
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title: '206. Reverse Linked List'
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description: Given the head of a singly linked list, reverse the list, and return the reversed list
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sidebar:
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label: 'Reverse Linked List'
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badge: 'Easy'
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---
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<Badge variant="accent">Linked List</Badge>
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::::warning
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A linked list can be reversed either iteratively or recursively. Could you implement both?
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::::
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### Example 1:
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- Input: `head = [1,2,3,4,5]`
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- Output: `[5,4,3,2,1]`
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### Example 2:
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- Input: `head = [1,2]`
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- Output: `[2,1]`
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### Example 3:
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- Input: `head = []`
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- Output: `[]`
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### Constraints:
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- The number of nodes in the list is the range [0, 5000].
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- `-5000 <= Node.val <= 5000`
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## Approach
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```mermaid
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flowchart TD
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S(["reverseList(head)"]) --> I["prev = None, curr = head"]
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I --> W{"curr?"}
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W -- no --> E(["return prev — the old tail is the new head"])
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W -- yes --> A["next_ = curr.next — save it before overwriting"]
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A --> B["curr.next = prev — flip the link"]
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B --> C["prev = curr; curr = next_ — step forward"]
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C --> W
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```
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## Solution
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```py
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# Definition for singly-linked list.
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# class ListNode:
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# def __init__(self, val=0, next=None):
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# self.val = val
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# self.next = next
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class Solution:
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def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
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prev = None
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curr = head
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while curr:
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next_ = curr.next
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curr.next = prev
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prev = curr
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curr = next_
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return prev
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```
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## Explanation
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<YouTube id="G0_I-ZF0S38" title="Reverse Linked List - Iterative AND Recursive - Leetcode 206 - Python" />
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