All Nodes Distance K in Binary Tree
<p>Given the <code>root</code> of a binary tree, the value of a target node <code>target</code>, and an integer <code>k</code>, return <em>an array of the values of all nodes that have a distance </em><code>k</code><em> from the target node.</em></p>

<p>You can return the answer in <strong>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/0800-0899/0863.All%20Nodes%20Distance%20K%20in%20Binary%20Tree/images/sketch0.png" style="width: 500px; height: 429px;" />
<pre>
<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], target = 5, k = 2
<strong>Output:</strong> [7,4,1]
Explanation: The nodes that are a distance 2 from the target node (with value 5) have values 7, 4, and 1.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1], target = 1, k = 3
<strong>Output:</strong> []
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 500]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 500</code></li>
	<li>All the values <code>Node.val</code> are <strong>unique</strong>.</li>
	<li><code>target</code> is the value of one of the nodes in the tree.</li>
	<li><code>0 &lt;= k &lt;= 1000</code></li>
</ul>
Tree; Depth-First Search; Breadth-First Search; Hash Table; Binary Tree
func distanceK(root *TreeNode, target *TreeNode, k int) []int {
	g := make(map[*TreeNode]*TreeNode)
	ans := []int{}

	var dfs func(node, fa *TreeNode)
	dfs = func(node, fa *TreeNode) {
		if node == nil {
			return
		}
		g[node] = fa
		dfs(node.Left, node)
		dfs(node.Right, node)
	}

	var dfs2 func(node, fa *TreeNode, k int)
	dfs2 = func(node, fa *TreeNode, k int) {
		if node == nil {
			return
		}
		if k == 0 {
			ans = append(ans, node.Val)
			return
		}
		for _, nxt := range []*TreeNode{node.Left, node.Right, g[node]} {
			if nxt != fa {
				dfs2(nxt, node, k-1)
			}
		}
	}

	dfs(root, nil)
	dfs2(target, nil, k)

	return ans
}
