Leetcode: 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

If we traverse the tree inorder, we just need to identify the 2 elements that are misplaced and swap their values. We also keep a third pointer that points to the previous element as it is needed to identify when an element is misplaced.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    TreeNode pre, node1, node2;   
    public void recoverTree(TreeNode root) {
        pre =null;
        node1 = null;
        node2 = null;
        inOrder(root);
        int temp = node1.val;
        node1.val = node2.val;
        node2.val = temp;
    }
    
    private void inOrder(TreeNode root)
    {
        if (root == null)
            return;
        inOrder(root.left);
        if(pre == null)
            pre = root;
        else if(node1 == null && pre.val > root.val)
        {
            node1 = pre;
            node2 = root;
        }
        else if(pre.val > root.val)
        {
            node2 = root;
        }
        pre = root;
        inOrder(root.right);
    }
}