题目

给你链表的头结点 head ，请将其按升序排列并返回 排序后的链表 。

示例 1：

输入：head = [4,2,1,3]
输出：[1,2,3,4]

示例 2：

输入：head = [-1,5,3,4,0]
输出：[-1,0,3,4,5]

示例 3：

输入：head = []
输出：[]

提示：

链表中节点的数目在范围 [0, 5 * 104] 内
-105 <= Node.val <= 105

解答

源代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        return mergeSort(head, null);
    }

    public ListNode mergeSort(ListNode head, ListNode tail) {
        if (head == tail) {
            return null;
        }

        if (head.next == tail) {
            head.next = null;
            return head;
        }

        ListNode fast = head, slow = head;

        while (fast != tail && fast.next != tail) {
            fast = fast.next.next;
            slow = slow.next;
        }

        ListNode head1 = mergeSort(head, slow);
        ListNode head2 = mergeSort(slow, tail);

        ListNode dummyNode = new ListNode(0);
        ListNode temp = dummyNode;

        while (head1 != null && head2 != null) {
            if (head1.val < head2.val) {
                temp.next = head1;
                head1 = head1.next;
            } else {
                temp.next = head2;
                head2 = head2.next;
            }

            temp = temp.next;
        }

        if (head1 != null) {
            temp.next = head1;
        } else {
            temp.next = head2;
        }

        return dummyNode.next;
    }
}