<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) {}
 * };
 */
class Solution {
    ListNode* merge(ListNode* head, ListNode* mid){
        ListNode* newHead = NULL, *temp = NULL, *curr = NULL;
        while(head || mid){
            if(!head){
                temp = mid;
                mid = mid->next;
                temp->next = NULL;
                if(!newHead) newHead = temp, curr = temp;
                else curr->next = temp, curr = curr->next;
            }
            else if(!mid){
                temp = head;
                head = head->next;
                temp->next = NULL;
                if(!newHead) newHead = temp, curr = temp;
                else curr->next = temp, curr = curr->next;
            }
            else{
                if(head->val <= mid->val){
                    temp = head;
                    head = head->next;
                    temp->next = NULL;
                    if(!newHead) newHead = temp, curr = temp;
                    else curr->next = temp, curr = curr->next;
                }
                else{
                    temp = mid;
                    mid = mid->next;
                    temp->next = NULL;
                    if(!newHead) newHead = temp, curr = temp;
                    else curr->next = temp, curr = curr->next;
                }
            }
        }
        return newHead;
    }
    ListNode* mergeSort(ListNode* head){
        if(!head || !head->next) return head;
        ListNode *slow = head, *fast = head, *prev = NULL;
        while(fast && fast->next){
            prev = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* mid = slow;
        prev->next = NULL;
        head = mergeSort(head);
        mid = mergeSort(mid);
        head = merge(head, mid);
        return head;
    }
public:
    ListNode* sortList(ListNode* head) {
        return mergeSort(head);
    }
};
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