Problem:

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

Reverse a singly linked list.

Solution 1:

Tracking prev / curr / next node

Time complexity: O(n)

Space complexity: O(1)

C++

// Author: Huahua
// Running time: 9 ms
class Solution {
public:
  ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* curr = head;
    ListNode* next;
    while (curr) {
      next = curr->next;
      curr->next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
};

Java

// Author: Huahua
// Running time: 0 ms
class Solution {
  public ListNode reverseList(ListNode head) {
    ListNode prev = null;
    ListNode curr = head;
    ListNode next;
    while (curr != null) {
      next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
}

Python 3

"""
Author: Huahua
Running time: 48 ms
"""
class Solution:
  def reverseList(self, head):
    prev, curr, nxt = None, head, None;    
    while curr:
      nxt, curr.next = curr.next, prev      
      prev, curr = curr, nxt
    return prev

The post 花花酱 LeetCode 206. Reverse Linked List appeared first on Huahua's Tech Road.