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:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void recoverTree(TreeNode root) {
        TreeNode pre=null, first=null, second=null;
        Stack<TreeNode> stack = new Stack<>();
        while(root!=null || !stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(pre!=null && pre.val>root.val){
                if(first==null){
                    first = pre;
                    second = root;
                } else {
                    second = root;
                }
            }
            pre = root;
            root = root.right;
        }
        int tmp = first.val;
        first.val = second.val;
        second.val = tmp;
    }
}