<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* findMid(ListNode* head){
    ListNode* slow = head;
    ListNode* fast = head->next;
    while(fast!=NULL && fast->next != NULL){
        slow = slow->next;
        fast = fast->next->next;
    }
    return slow;
}
// ListNode* merge(ListNode* &left, ListNode* &right){
//     ListNode* p1 =left;
//     ListNode* p2 =right;

//     ListNode* dummyNode = new ListNode(-1);
//     ListNode* p3 = dummyNode;

//     while(p1!=NULL && p2!=NULL){
//         if(p1->val < p2->val){
//             p3->next = p1;
//             p1 = p1->next;
//         }
//         else{
//             p3->next = p2;
//             p2 = p2->next;
//         }
//     p3 = p3->next;
//     }
//     while(p1!=NULL){
//         p3->next = p1;
//         p1 = p1->next;
//         p3 = p3->next;
//     }
//     while(p2!=NULL){
//         p3->next = p2;
//         p2 = p2->next;
//         p3 = p3->next;
//     }
//     return dummyNode->next;
// }
    ListNode* merge(ListNode* &left, ListNode* &right){
   //base case
   if(left==NULL){
       return right;
   }
   if(right == NULL){
       return left;
   }
   ListNode* result;
   if(left->val < right ->val){
       result = left;
       result->next = merge(left->next, right);
   }
   else{
       result = right;
       result->next = merge(left, right->next);
   }
   return result;
}
    ListNode* sortList(ListNode* head) {
        //USING MERGE SORT
        if(head == 0 || head->next == 0){
            return head;
        }
        ListNode* mid = findMid(head);
        ListNode* left = head;
        ListNode* right = mid->next;
        mid->next = NULL;

        //sort RE
        left = sortList(left);
        right = sortList(right);

        //merge both left and right LLS
        ListNode* mergedLL = merge(left, right);
        return mergedLL;
    }
};
25793