【LeetCode】206. Reverse Linked List 解題報告

發表於 更新於 分類於

206. Reverse Linked List / Easy

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

Solution

思路

如果要 in-place 反轉整個 linked list，就必須要每一步都將 node 之間的指標反轉。
至少會需要三個指標，一個指向前面，一個指向後面，另一個則是目前節點的指標。
直接看程式碼，很直白。

效能

Complexity

  • Time Complexity: O(N), where N is the length of the linked list
  • Space Complexity: O(1)

LeetCode Result

Code

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head) return nullptr;
        if(!head->next) return head;
        
        ListNode *prev = nullptr, *next = head->next, *curr = head;
        while(curr) {
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
};