876. Middle of the Linked List

Leetcode 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.

分析

使用快指针和慢指针。一开始，它们都指向链表头部，快指针每次移动两步，慢指针每次移动一步。当快指针不能再移动时，也就是到达链表尾部时，慢指针的位置就是链表的中间位置。

public ListNode middleNode(ListNode head) {
    ListNode fast = head, slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }
    return slow;
}