<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* mergeLists(ListNode* first, ListNode* second) {
        if (first == nullptr)
            return second;
        if (second == nullptr)
            return first;        
        if (first->val > second->val) {
            ListNode* tmp = first;
            first = second;
            second = tmp;
        }
        
        ListNode* next = first, *secondNext = second;
        while (next->next != nullptr && secondNext != nullptr)
        {
            if (next->next->val < secondNext->val)
                next = next->next;
            else {
                ListNode* tmp = next->next;
                next->next = secondNext;
                secondNext = secondNext->next;
                next->next->next = tmp;
                next = next->next;
            }
        }
        if (secondNext != nullptr)
            next->next = secondNext;
        return first;
    }

    struct Data {
        int number = 0;
        ListNode* node = nullptr;
    };

    ListNode* sortList(ListNode* head) {
        std::vector<Data> stack;
        stack.reserve(1000);
        while (head != nullptr)
        {            
            int count = stack.size();
            if (count > 1)
            {
                Data& last = stack[count-1];
                Data& prev = stack[count-2];
                if (last.number == prev.number)
                {
                    prev.node = mergeLists(prev.node, last.node);
                    prev.number *= 2;
                    stack.pop_back();
                }
            }
            if (head != nullptr) 
            {                
                ListNode* tmp = head->next;
                head->next = nullptr;
                Data d;
                d.number = 1;
                d.node = head;              
                stack.push_back(d);
                head = tmp;
            }
        }

        while (stack.size() > 1)
        {
            int count = stack.size();
            Data& last = stack[count-1];
            Data& prev = stack[count-2];            
            prev.node = mergeLists(prev.node, last.node);
            prev.number *= 2;
            stack.pop_back();
        }
        if (stack.empty())
            return nullptr;
        return stack[0].node;
    }
};
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