Recover Binary Search Tree
<p>You are given the <code>root</code> of a binary search tree (BST), where the values of <strong>exactly</strong> two nodes of the tree were swapped by mistake. <em>Recover the tree without changing its structure</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0099.Recover%20Binary%20Search%20Tree/images/recover1.jpg" style="width: 422px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [1,3,null,null,2]
<strong>Output:</strong> [3,1,null,null,2]
<strong>Explanation:</strong> 3 cannot be a left child of 1 because 3 &gt; 1. Swapping 1 and 3 makes the BST valid.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0099.Recover%20Binary%20Search%20Tree/images/recover2.jpg" style="width: 581px; height: 302px;" />
<pre>
<strong>Input:</strong> root = [3,1,4,null,null,2]
<strong>Output:</strong> [2,1,4,null,null,3]
<strong>Explanation:</strong> 2 cannot be in the right subtree of 3 because 2 &lt; 3. Swapping 2 and 3 makes the BST valid.
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[2, 1000]</code>.</li>
	<li><code>-2<sup>31</sup> &lt;= Node.val &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> A solution using <code>O(n)</code> space is pretty straight-forward. Could you devise a constant <code>O(1)</code> space solution?
Tree; Depth-First Search; Binary Search Tree; Binary Tree
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {void} Do not return anything, modify root in-place instead.
 */
var recoverTree = function (root) {
    let prev = null;
    let first = null;
    let second = null;
    function dfs(root) {
        if (!root) {
            return;
        }
        dfs(root.left);
        if (prev && prev.val > root.val) {
            if (!first) {
                first = prev;
            }
            second = root;
        }
        prev = root;
        dfs(root.right);
    }
    dfs(root);
    const t = first.val;
    first.val = second.val;
    second.val = t;
};
