<p>Given the <code>head</code> of a linked list, return <em>the list after sorting it in <strong>ascending order</strong></em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/14/sort_list_1.jpg" style="width: 450px; height: 194px;" />
<pre>
<strong>Input:</strong> head = [4,2,1,3]
<strong>Output:</strong> [1,2,3,4]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/14/sort_list_2.jpg" style="width: 550px; height: 184px;" />
<pre>
<strong>Input:</strong> head = [-1,5,3,4,0]
<strong>Output:</strong> [-1,0,3,4,5]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> head = []
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the list is in the range <code>[0, 5 * 10<sup>4</sup>]</code>.</li>
	<li><code>-10<sup>5</sup> &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Can you sort the linked list in <code>O(n logn)</code> time and <code>O(1)</code> memory (i.e. constant space)?</p>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

using Node = ListNode;
class Solution {
public:
    Node* thead, *cur, *tmp;

    int lengthList(ListNode* node){
        if (node == nullptr) return 0;
        return 1 + lengthList(node->next); 
    }

    Node* merge(Node* node1, Node* node2){
        thead = cur = nullptr;
        while (node1 != nullptr && node2 != nullptr){
            if (node1->val <= node2->val){
                tmp = node1;
                node1 = node1->next;
            } else {
                tmp = node2;
                node2 = node2->next;
            }
            if (thead == nullptr){
                thead = cur = tmp;
            } else {
                cur->next = tmp;
                cur = tmp;
            }
            cur->next = nullptr;
        }
        while (node1 != nullptr){
            tmp = node1;
            node1 = node1->next;
            if (thead == nullptr){
                thead = cur = tmp;
            } else {
                cur->next = tmp;
                cur = tmp;
            }
            cur->next = nullptr;
        }
        while (node2 != nullptr){
            tmp = node2;
            node2 = node2->next;
            if (thead == nullptr){
                thead = cur = tmp;
            } else {
                cur->next = tmp;
                cur = tmp;
            }
            cur->next = nullptr;
        }
        return thead;
    }

    Node* sort(Node* head, int n){
        if (n <= 1 || head == nullptr) return head;
        Node* cur = head, *tmp = nullptr;
        for (int i = 1; i <= n/2; i++){
            tmp = cur;
            cur = cur->next;
        }
        tmp->next = nullptr;
        return merge(sort(head, n/2), sort(cur, n - n/2));
    }

    ListNode* sortList(ListNode* head) {
        int n = lengthList(head);
        return sort(head, n);
    }
};
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