Symmetric Tree
<p>Given the <code>root</code> of a binary tree, <em>check whether it is a mirror of itself</em> (i.e., symmetric around its center).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0101.Symmetric%20Tree/images/symtree1.jpg" style="width: 354px; height: 291px;" />
<pre>
<strong>Input:</strong> root = [1,2,2,3,4,4,3]
<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/0100-0199/0101.Symmetric%20Tree/images/symtree2.jpg" style="width: 308px; height: 258px;" />
<pre>
<strong>Input:</strong> root = [1,2,2,null,3,null,3]
<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, 1000]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> Could you solve it both recursively and iteratively?
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 isSymmetric(root: TreeNode | null): boolean {
    const dfs = (root1: TreeNode | null, root2: TreeNode | null): boolean => {
        if (root1 === root2) {
            return true;
        }
        if (!root1 || !root2 || root1.val !== root2.val) {
            return false;
        }
        return dfs(root1.left, root2.right) && dfs(root1.right, root2.left);
    };
    return dfs(root.left, root.right);
}
