Find All The Lonely Nodes
<p>In a binary tree, a <strong>lonely</strong> node is a node that is the only child of its parent node. The root of the tree is not lonely because it does not have a parent node.</p>

<p>Given the <code>root</code> of a binary tree, return <em>an array containing the values of all lonely nodes</em> in the tree. Return the list <strong>in any order</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/1400-1499/1469.Find%20All%20The%20Lonely%20Nodes/images/e1.png" style="width: 203px; height: 202px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,null,4]
<strong>Output:</strong> [4]
<strong>Explanation:</strong> Light blue node is the only lonely node.
Node 1 is the root and is not lonely.
Nodes 2 and 3 have the same parent and are not lonely.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1400-1499/1469.Find%20All%20The%20Lonely%20Nodes/images/e2.png" style="width: 442px; height: 282px;" />
<pre>
<strong>Input:</strong> root = [7,1,4,6,null,5,3,null,null,null,null,null,2]
<strong>Output:</strong> [6,2]
<strong>Explanation:</strong> Light blue nodes are lonely nodes.
Please remember that order doesn&#39;t matter, [2,6] is also an acceptable answer.
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1400-1499/1469.Find%20All%20The%20Lonely%20Nodes/images/tree.png" style="width: 363px; height: 202px;" />
<pre>
<strong>
Input:</strong> root = [11,99,88,77,null,null,66,55,null,null,44,33,null,null,22]
<strong>Output:</strong> [77,55,33,66,44,22]
<strong>Explanation:</strong> Nodes 99 and 88 share the same parent. Node 11 is the root.
All other nodes are lonely.
</pre>

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

<ul>
	<li>The number of nodes in the <code>tree</code> is in the range <code>[1, 1000].</code></li>
	<li><code>1 &lt;= Node.val &lt;= 10<sup>6</sup></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 getLonelyNodes(root: TreeNode | null): number[] {
    const ans: number[] = [];
    const dfs = (root: TreeNode | null) => {
        if (!root || root.left === root.right) {
            return;
        }
        if (!root.left) {
            ans.push(root.right.val);
        }
        if (!root.right) {
            ans.push(root.left.val);
        }
        dfs(root.left);
        dfs(root.right);
    };
    dfs(root);
    return ans;
}
