<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) {}
 * };
 */

const int N = 5e4 + 2;
int a[N];

int partition(int low, int high) {
    int i = low - 1;
    int pivot_index = low + rand() % (high - low + 1);
    swap(a[pivot_index], a[high]);
    int pivot = a[high];
    for(int j = low;j <= high;j++) {
        if(a[j] <= pivot) {
            swap(a[++i], a[j]);
        }
    }
    return i;
}

void quicksort(int low, int high) {
    if(low >= high)
        return;
    int mid = partition(low, high);
    quicksort(low, mid - 1);
    quicksort(mid + 1, high);
}

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (head == nullptr)
            return nullptr;
        int n = 0;
        for(auto node = head;node;node = node->next) {
            a[++n] = node->val;
        }
        quicksort(1, n);
        ListNode* newHead = new ListNode(a[1]);
        ListNode* tail = newHead;
        for(int i = 2;i <= n;i++) {
            tail->next = new ListNode(a[i]);
            tail = tail->next;
        }
        return newHead;
    }
};
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