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.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int x;

    public boolean isUnivalTree(TreeNode root) {
        x = root.val;
        return dfs(root);
    }

    private boolean dfs(TreeNode root) {
        if (root == null) {
            return true;
        }
        return root.val == x && dfs(root.left) && dfs(root.right);
    }
}
