<p>You are given the head of a singly linked-list. The list can be represented as:</p>

<pre>
L<sub>0</sub> &rarr; L<sub>1</sub> &rarr; &hellip; &rarr; L<sub>n - 1</sub> &rarr; L<sub>n</sub>
</pre>

<p><em>Reorder the list to be on the following form:</em></p>

<pre>
L<sub>0</sub> &rarr; L<sub>n</sub> &rarr; L<sub>1</sub> &rarr; L<sub>n - 1</sub> &rarr; L<sub>2</sub> &rarr; L<sub>n - 2</sub> &rarr; &hellip;
</pre>

<p>You may not modify the values in the list&#39;s nodes. Only nodes themselves may be changed.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/04/reorder1linked-list.jpg" style="width: 422px; height: 222px;" />
<pre>
<strong>Input:</strong> head = [1,2,3,4]
<strong>Output:</strong> [1,4,2,3]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/09/reorder2-linked-list.jpg" style="width: 542px; height: 222px;" />
<pre>
<strong>Input:</strong> head = [1,2,3,4,5]
<strong>Output:</strong> [1,5,2,4,3]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the list is in the range <code>[1, 5 * 10<sup>4</sup>]</code>.</li>
	<li><code>1 &lt;= Node.val &lt;= 1000</code></li>
</ul>

class Solution {
public:
    void reorderList(ListNode* head) {
        if (!head) return;

        ListNode* p1 = head;
        int c = 0;
        vector<pair<int, ListNode*>> mp;

        // Store the nodes in a vector
        while (p1 != NULL) {
            mp.push_back({p1->val, p1});
            p1 = p1->next;
            c++;
        }

        // Reorder the list
        int i = 0, j = c - 1;
        p1 = head;

        while (i < j) {
            ListNode* p2 = mp[i].second->next; // Save the next node
            mp[i].second->next = mp[j].second; // Point to the node from the end
            mp[j].second->next = p2;           // Point the node from end to next node of the ith node
            i++;
            j--;
        }

        mp[i].second->next = NULL;
    }
};

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