<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 {
public:

    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if(list1 == NULL){
            return  list2;
        }
        if(list2 == NULL){
            return list1;
        }

        ListNode* head = NULL;
        ListNode* temp = head;
        ListNode* temp1 = list1;
        ListNode* temp2 = list2;

        while(temp1!=NULL && temp2!=NULL){
            if(temp1->val < temp2->val){
                if(head == NULL){
                    head = new ListNode(temp1->val);
                    temp = head;
                }
                else{
                    temp->next = temp1;
                    temp = temp->next;
                }
                temp1 = temp1->next;
            }
            else{
                if(head == NULL){
                    head = new ListNode(temp2->val);
                    temp = head;
                }
                else{
                    temp->next = temp2;
                    temp = temp->next;
                }
                temp2 = temp2->next;
            }
        }

        if(temp1!=NULL){
           temp->next = temp1;
            //temp = temp->next;
            //temp1 = temp1->next;
        }
        if(temp2!=NULL){
           temp->next = temp2;
            //temp = temp->next;
            //temp2 = temp2->next;
        }

        return head;
    }
    ListNode* middleNode(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head;

        while(fast->next != NULL && fast->next->next!= NULL){
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
    ListNode* sortList(ListNode* head) {
        if(!head || !head->next){
            return head;
        }
        ListNode* temp1 = head;
        ListNode* mid = middleNode(temp1);
        ListNode* temp2 = mid->next;
        mid->next = NULL;

        ListNode* a = sortList(temp1);
        ListNode* b = sortList(temp2);
        return mergeTwoLists(a,b);

    }
};
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