<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:
    void merge(vector<int>& arr, int s, int mid, int e) {
        int left = s;
        int right = mid + 1;
        int temp[e+1];
        int k = 0;
        while (left <= mid && right <= e) {
            if (arr[left] < arr[right]) {
                temp[k++] = arr[left++];
            } else {
                temp[k++] = arr[right++];
            }
        }
        while (left <= mid) {
            temp[k++] = arr[left++];
        }
        while (right <= e) {
            temp[k++] = arr[right++];
        }
        // int r=0;
        for (int i = s; i <= e; i++) { // s-s=0, s+1-s=1
            arr[i] = temp[i - s];
        }
    }
    void mergeSort(vector<int>& arr, int s, int e) {
        if (s >= e) {
            return;
        }
        int mid = (s + e) / 2;
        mergeSort(arr, s, mid);
        mergeSort(arr, mid + 1, e);
        merge(arr, s, mid, e);
    }
    ListNode* sortList(ListNode* head) {
        vector<int> v;
        ListNode* temp = head;
        while(temp != NULL){
            v.push_back(temp->val);
            temp=temp->next;
        }
        mergeSort(v, 0 ,v.size()-1);
        ListNode* curr = head;
        int k=0;
        while(curr != NULL){
            curr->val = v[k++];
            curr=curr->next;
         }
         return head;
        
    }
};
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