<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) {}
 * };
 */
#include <set>

struct ListNodeComparator {
    bool operator()(const ListNode* lhs, const ListNode* rhs) const {
        return lhs->val < rhs->val;
    }
};



class Solution {
public:
    ListNode* sortList(ListNode* head) {
        
        std::multiset<ListNode*, ListNodeComparator> custom_set;
        auto curr = head;
        while(curr)
        {
            custom_set.insert(curr);
            curr = curr->next;
        }
        ListNode dummy(0, nullptr);
        curr = &dummy;
        for(auto it = custom_set.begin(); it != custom_set.end(); ++it)
        {
            curr->next = *it;
            curr = *it;
        }
        curr->next = nullptr;

        return dummy.next;

        if(!head) return head;

        auto outer = head;
        while(outer)
        {
            auto inner = head;
            while(inner && inner != outer)
            {
                if(outer->val < inner->val)
                {
                    std::swap(outer->val, inner->val);
                }
                inner = inner->next;
            }
            outer = outer->next;

        }
        return head;
    }
};
71976