LeetCode 148 排序链表Java

Raymond于2019.12.04晚，第一篇CSDN博客。

题目描述

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4
示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/sort-list
参考：https://blog.csdn.net/mine_song/article/details/69831827

代码

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

        // 找到中间节点
        ListNode fast = head;
        ListNode slow = head;
        while (fast.next != null){
            fast = fast.next.next;
            if (fast == null){
                break;
            }
            slow = slow.next;
        }
        ListNode right = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        right = sortList(right);
        return mergeTwoList(left, right);
    }

    // 递归实现两个链的合并
    public ListNode mergeTwoList(ListNode left, ListNode right){
        if (left == null) return right;
        if (right == null) return left;

        ListNode res = null;
        if (left.val <= right.val){
            res = left;
            left.next = mergeTwoList(left.next, right);
        }else {
            res = right;
            right.next = mergeTwoList(left, right.next);
        }
        return res;
    }
}