Delete Nodes And Return Forest
<p>Given the <code>root</code> of a binary tree, each node in the tree has a distinct value.</p>

<p>After deleting all nodes with a value in <code>to_delete</code>, we are left with a forest (a disjoint union of trees).</p>

<p>Return the roots of the trees in the remaining forest. You may return the result in any order.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1100-1199/1110.Delete%20Nodes%20And%20Return%20Forest/images/screen-shot-2019-07-01-at-53836-pm.png" style="width: 237px; height: 150px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,4,5,6,7], to_delete = [3,5]
<strong>Output:</strong> [[1,2,null,4],[6],[7]]
</pre>

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

<pre>
<strong>Input:</strong> root = [1,2,4,null,3], to_delete = [3]
<strong>Output:</strong> [[1,2,4]]
</pre>

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

<ul>
	<li>The number of nodes in the given tree is at most <code>1000</code>.</li>
	<li>Each node has a distinct value between <code>1</code> and <code>1000</code>.</li>
	<li><code>to_delete.length &lt;= 1000</code></li>
	<li><code>to_delete</code> contains distinct values between <code>1</code> and <code>1000</code>.</li>
</ul>
Tree; Depth-First Search; Array; Hash Table; Binary Tree
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func delNodes(root *TreeNode, to_delete []int) (ans []*TreeNode) {
	s := make([]bool, 1001)
	for _, x := range to_delete {
		s[x] = true
	}
	var dfs func(*TreeNode) *TreeNode
	dfs = func(root *TreeNode) *TreeNode {
		if root == nil {
			return nil
		}
		root.Left = dfs(root.Left)
		root.Right = dfs(root.Right)
		if !s[root.Val] {
			return root
		}
		if root.Left != nil {
			ans = append(ans, root.Left)
		}
		if root.Right != nil {
			ans = append(ans, root.Right)
		}
		return nil
	}
	if dfs(root) != nil {
		ans = append(ans, root)
	}
	return
}