[LeetCode] 876. 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.

链表的中间结点。

给你单链表的头结点 head ，请你找出并返回链表的中间结点。
如果有两个中间结点，则返回第二个中间结点。

思路

思路是快慢指针，快指针停下的时候，慢指针所在的位置即是所求。

复杂度

时间O(n)
空间O(1)

代码

Java实现

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

JavaScript实现

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var middleNode = function(head) {
    let slow = head;
    let fast = head;
    while (fast !== null && fast.next !== null) {
        slow = slow.next;
        fast = fast.next.next;
    }
    return slow;
};

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