<p>Given the <code>root</code> of a binary tree, return <em>the level order traversal of its nodes&#39; values</em>. (i.e., from left to right, level by level).</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],[9,20],[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>-1000 &lt;= Node.val &lt;= 1000</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>> levelOrder(TreeNode *root)
    {
        vector<vector<int>> level;
        bfs(root, level);
        return level;
    }

    void bfs(TreeNode *root, vector<vector<int>> &level)
    {   
        if (root)
        {
            queue<vector<TreeNode *>> order;
        vector<TreeNode *> first;
        first.push_back(root);
        order.push(first);
        vector<int> firstint;
        firstint.push_back(root->val);
        level.push_back(firstint);
        while (!order.empty())
        {
            vector<int> nextlevel;
            vector<TreeNode *> nextroots;
            vector<TreeNode *> prevlevel = order.front();
            for (auto node : prevlevel)
            {
                if (node)
                {
                    if (node->left)
                    {
                        nextlevel.push_back(node->left->val);
                        nextroots.push_back(node->left);
                    }
                    if (node->right)
                    {
                        nextlevel.push_back(node->right->val);
                        nextroots.push_back(node->right);
                    }
                }
            }
            order.pop();
            if (nextroots.size()>0)
            {
                order.push(nextroots);
                level.push_back(nextlevel);
            }
        }
        }
        
    }

};
15600