Create Binary Tree From Descriptions
<p>You are given a 2D integer array <code>descriptions</code> where <code>descriptions[i] = [parent<sub>i</sub>, child<sub>i</sub>, isLeft<sub>i</sub>]</code> indicates that <code>parent<sub>i</sub></code> is the <strong>parent</strong> of <code>child<sub>i</sub></code> in a <strong>binary</strong> tree of <strong>unique</strong> values. Furthermore,</p>

<ul>
	<li>If <code>isLeft<sub>i</sub> == 1</code>, then <code>child<sub>i</sub></code> is the left child of <code>parent<sub>i</sub></code>.</li>
	<li>If <code>isLeft<sub>i</sub> == 0</code>, then <code>child<sub>i</sub></code> is the right child of <code>parent<sub>i</sub></code>.</li>
</ul>

<p>Construct the binary tree described by <code>descriptions</code> and return <em>its <strong>root</strong></em>.</p>

<p>The test cases will be generated such that the binary tree is <strong>valid</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2100-2199/2196.Create%20Binary%20Tree%20From%20Descriptions/images/example1drawio.png" style="width: 300px; height: 236px;" />
<pre>
<strong>Input:</strong> descriptions = [[20,15,1],[20,17,0],[50,20,1],[50,80,0],[80,19,1]]
<strong>Output:</strong> [50,20,80,15,17,19]
<strong>Explanation:</strong> The root node is the node with value 50 since it has no parent.
The resulting binary tree is shown in the diagram.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2100-2199/2196.Create%20Binary%20Tree%20From%20Descriptions/images/example2drawio.png" style="width: 131px; height: 300px;" />
<pre>
<strong>Input:</strong> descriptions = [[1,2,1],[2,3,0],[3,4,1]]
<strong>Output:</strong> [1,2,null,null,3,4]
<strong>Explanation:</strong> The root node is the node with value 1 since it has no parent.
The resulting binary tree is shown in the diagram.
</pre>

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

<ul>
	<li><code>1 &lt;= descriptions.length &lt;= 10<sup>4</sup></code></li>
	<li><code>descriptions[i].length == 3</code></li>
	<li><code>1 &lt;= parent<sub>i</sub>, child<sub>i</sub> &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= isLeft<sub>i</sub> &lt;= 1</code></li>
	<li>The binary tree described by <code>descriptions</code> is valid.</li>
</ul>
Tree; Array; Hash Table; Binary Tree
/**
 * 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:
    TreeNode* createBinaryTree(vector<vector<int>>& descriptions) {
        unordered_map<int, TreeNode*> nodes;
        unordered_set<int> children;
        for (const auto& d : descriptions) {
            int parent = d[0], child = d[1], isLeft = d[2];
            if (!nodes.contains(parent)) {
                nodes[parent] = new TreeNode(parent);
            }
            if (!nodes.contains(child)) {
                nodes[child] = new TreeNode(child);
            }
            if (isLeft) {
                nodes[parent]->left = nodes[child];
            } else {
                nodes[parent]->right = nodes[child];
            }
            children.insert(child);
        }
        for (const auto& [k, v] : nodes) {
            if (!children.contains(k)) {
                return v;
            }
        }
        return nullptr;
    }
};