#yyds干货盘点# LeetCode 腾讯精选练习 50 题：排序链表

Posted 2022-11-12

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：[]

代码实现：

class Solution 
    public ListNode sortList(ListNode head) 
        if (head == null) 
            return head;
        
        int length = 0;
        ListNode node = head;
        while (node != null) 
            length++;
            node = node.next;
        
        ListNode dummyHead = new ListNode(0, head);
        for (int subLength = 1; subLength < length; subLength <<= 1) 
            ListNode prev = dummyHead, curr = dummyHead.next;
            while (curr != null) 
                ListNode head1 = curr;
                for (int i = 1; i < subLength && curr.next != null; i++) 
                    curr = curr.next;
                
                ListNode head2 = curr.next;
                curr.next = null;
                curr = head2;
                for (int i = 1; i < subLength && curr != null && curr.next != null; i++) 
                    curr = curr.next;
                
                ListNode next = null;
                if (curr != null) 
                    next = curr.next;
                    curr.next = null;
                
                ListNode merged = merge(head1, head2);
                prev.next = merged;
                while (prev.next != null) 
                    prev = prev.next;
                
                curr = next;
            
        
        return dummyHead.next;
    

    public ListNode merge(ListNode head1, ListNode head2) 
        ListNode dummyHead = new ListNode(0);
        ListNode temp = dummyHead, temp1 = head1, temp2 = head2;
        while (temp1 != null && temp2 != null) 
            if (temp1.val <= temp2.val) 
                temp.next = temp1;
                temp1 = temp1.next;
             else 
                temp.next = temp2;
                temp2 = temp2.next;
            
            temp = temp.next;
        
        if (temp1 != null) 
            temp.next = temp1;
         else if (temp2 != null) 
            temp.next = temp2;
        
        return dummyHead.next;