<p>Given the <code>root</code> of a binary tree, return <em>the zigzag level order traversal of its nodes&#39; values</em>. (i.e., from left to right, then right to left for the next level and alternate between).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg" style="width: 277px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> [[3],[20,9],[15,7]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1]
<strong>Output:</strong> [[1]]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = []
<strong>Output:</strong> []
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (root == nullptr) return ans;
        deque<TreeNode*> dq; // :P
        dq.push_back(root);
        bool front = true; // :D
        queue<TreeNode*> lvlq;
        while (!dq.empty()) {
            ans.push_back(vector<int>());
            auto level = ans.end() - 1;
            while (!dq.empty()) {
                TreeNode* bepis = front ? dq.front() : dq.back();
                if (front) dq.pop_front(); else dq.pop_back();
                (*level).push_back(bepis->val);
                if (front) {
                    if (bepis->left != nullptr) lvlq.push(bepis->left);
                    if (bepis->right != nullptr) lvlq.push(bepis->right);
                } else {
                    if (bepis->right != nullptr) lvlq.push(bepis->right);
                    if (bepis->left != nullptr) lvlq.push(bepis->left);
                }
            }
            while (!lvlq.empty()) {
                if (front) dq.push_back(lvlq.front());
                else dq.push_front(lvlq.front());
                lvlq.pop();
            }
            front ^= 1;
        }
        return ans;
    }
};
13000