LeetCode.876.Middle of the Linked List 链表的中间结点

题目描述

876 Middle of the Linked List https://leetcode-cn.com/problems/middle-of-the-linked-list/

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 and 100.

解题过程

双指针，快指针每次走2步，慢指针每次走1步，快指针到尾部时慢指针在中间，写完后具体考虑下细节即可。

时间复杂度 O(n)，空间复杂度 O(1)

private static class SolutionV2020 {
    public ListNode middleNode(ListNode head) {
        if (null == head || null == head.next) {
            return head;
        }
        ListNode slow = head, fast = head;
        // 快指针每次走2步，慢指针每次走1步
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
}

GitHub代码

algorithms/leetcode/leetcode/_876_MiddleOfLinkedList.java https://github.com/masikkk/algorithms/blob/master/leetcode/leetcode/_876_MiddleOfLinkedList.java