Increasing Order Search Tree
<p>Given the <code>root</code> of a binary search tree, rearrange the tree in <strong>in-order</strong> so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only one right child.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0800-0899/0897.Increasing%20Order%20Search%20Tree/images/ex1.jpg" style="width: 600px; height: 350px;" />
<pre>
<strong>Input:</strong> root = [5,3,6,2,4,null,8,1,null,null,null,7,9]
<strong>Output:</strong> [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0800-0899/0897.Increasing%20Order%20Search%20Tree/images/ex2.jpg" style="width: 300px; height: 114px;" />
<pre>
<strong>Input:</strong> root = [5,1,7]
<strong>Output:</strong> [1,null,5,null,7]
</pre>

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

<ul>
	<li>The number of nodes in the given tree will be in the range <code>[1, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 1000</code></li>
</ul>
Stack; Tree; Depth-First Search; Binary Search Tree; Binary Tree
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func increasingBST(root *TreeNode) *TreeNode {
	dummy := &TreeNode{Val: 0, Right: root}
	prev := dummy
	var dfs func(root *TreeNode)
	dfs = func(root *TreeNode) {
		if root == nil {
			return
		}
		dfs(root.Left)
		prev.Right = root
		root.Left = nil
		prev = root
		dfs(root.Right)
	}
	dfs(root)
	return dummy.Right
}