LeetCode 876. Middle of the Linked List
原题链接在这里:https://leetcode.com/problems/middle-of-the-linked-list/
题目:
Given a non-empty, singly linked list with head node head
, return a middle node of linked list.
If there are two middle nodes, return the second middle node.
Example 1:
Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3. (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
Example 2:
Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.
Note:
- The number of nodes in the given list will be between
1
and100
.
题解:
walker每次跳一步. runner每次跳两步. runner 跳到尾时, walker就是中点.
list偶数长度时, 中点找中间两个前者时, while的终止条件到是(runner != null && runner.next != null).
找后者时, while的终止条件是(runner != null && runner.next != null).
Time Complexity: O(n).
Space: O(1).
AC Java:
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * int val; 5 * ListNode next; 6 * ListNode(int x) { val = x; } 7 * } 8 */ 9 class Solution { 10 public ListNode middleNode(ListNode head) { 11 if(head == null || head.next == null){ 12 return head; 13 } 14 15 ListNode walker = head; 16 ListNode runner = head; 17 while(runner != null && runner.next != null){ 18 walker = walker.next; 19 runner = runner.next.next; 20 } 21 22 return walker; 23 } 24 }
AC C++:
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode() : val(0), next(nullptr) {} 7 * ListNode(int x) : val(x), next(nullptr) {} 8 * ListNode(int x, ListNode *next) : val(x), next(next) {} 9 * }; 10 */ 11 class Solution { 12 public: 13 ListNode* middleNode(ListNode* head) { 14 if(!head || !head->next){ 15 return head; 16 } 17 18 ListNode* walker = head; 19 ListNode* runner = head; 20 while(runner && runner->next){ 21 walker = walker->next; 22 runner = runner->next->next; 23 } 24 25 return walker; 26 } 27 };
AC Python:
1 # Definition for singly-linked list. 2 # class ListNode: 3 # def __init__(self, val=0, next=None): 4 # self.val = val 5 # self.next = next 6 class Solution: 7 def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]: 8 walker = head 9 runner = head 10 while runner and runner.next: 11 walker = walker.next 12 runner = runner.next.next 13 return walker