Reverse Linked List

Description

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?

Code

使用 Palindrome Linked List 中學到的技巧，將 linked list 反轉。

Time Complexity: $O(N)$, Space Complexity: $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* reverseList(struct ListNode* head) {
    if(!head) return NULL;
    if(!head->next) return head;
    struct ListNode* prev = NULL;
    struct ListNode* cur = head;
    while(cur) {
        struct ListNode* next = cur->next;
        cur->next = prev;
        prev = cur;
        cur = next;
    }
    return prev;
}

/**
 * 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;
        ListNode* prev = nullptr;
        while(cur) {
            ListNode* temp = cur->next;
            cur->next = prev;
            prev = cur;
            cur = temp;
        }
 
        return prev;
 
    }
};

Source

• Reverse Linked List - LeetCode