206. Reverse Linked List

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206. Reverse Linked List

Description

Difficulty: Easy

Related Topics: Linked List, Recursion

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

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Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

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Input: head = [1,2]
Output: [2,1]

Example 3:

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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?

Hints/Notes

  • 2023/08/02
  • For recursive solution, only think how to handle then end case and how to handle a new tail
  • 0x3F’s solution(checked)
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class Solution {
public:
    ListNode* reverse(ListNode* head) {
        return nullptr;
    }
}

Solution

Iterative solution:

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode *cur = head, *prev = nullptr;
        while (cur) {
            ListNode* nxt = cur->next;
            cur->next = prev;
            prev = cur;
            cur = nxt;
        }
        return prev;
    }
};

Language: C++

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (head == nullptr || head->next == nullptr) {
            return head;
        }
        ListNode* newHead = reverseList(head->next);
        head->next->next = head;
        // this is only useful for the head as well
        head->next = nullptr;
        return newHead;
    }
};