Univalued Binary Tree
<p>A binary tree is <strong>uni-valued</strong> if every node in the tree has the same value.</p>

<p>Given the <code>root</code> of a binary tree, return <code>true</code><em> if the given tree is <strong>uni-valued</strong>, or </em><code>false</code><em> otherwise.</em></p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0900-0999/0965.Univalued%20Binary%20Tree/images/unival_bst_1.png" style="width: 265px; height: 172px;" />
<pre>
<strong>Input:</strong> root = [1,1,1,1,1,null,1]
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0900-0999/0965.Univalued%20Binary%20Tree/images/unival_bst_2.png" style="width: 198px; height: 169px;" />
<pre>
<strong>Input:</strong> root = [2,2,2,5,2]
<strong>Output:</strong> false
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt; 100</code></li>
</ul>
Tree; Depth-First Search; Breadth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function isUnivalTree(root: TreeNode | null): boolean {
    const x = root!.val;
    const dfs = (root: TreeNode | null): boolean => {
        if (!root) {
            return true;
        }
        return root.val === x && dfs(root.left) && dfs(root.right);
    };
    return dfs(root);
}
