# 99. Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Note: A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

## Solution <a href="#solution" id="solution"></a>

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private TreeNode first;
    private TreeNode second;
    private TreeNode prev = new TreeNode(Integer.MIN_VALUE);

    public void recoverTree(TreeNode root) {
        inOrder(root);

        int temp = first.val;
        first.val = second.val;
        second.val = temp;
    }

    private void inOrder(TreeNode root) {
        if (root == null) {
            return;
        }

        inOrder(root.left);
        if (first == null && prev.val > root.val) {
            first = prev;
        }
        if (first != null && prev.val > root.val) {
            second = root;
        }
        prev = root;
        inOrder(root.right);
    }
}
```
