[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].java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void recoverTree(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode first = null;
        TreeNode second = null;
        TreeNode last = null;
        TreeNode run = root;

        while(run != null){
            stack.add(run);
            run = run.left;
        }
        while(!stack.isEmpty()){
            run = stack.pop();

            if(last == null) last = run;
            else if(run.val < last.val){
                if(first == null){
                    first = last;
                    second = run;
                }
                else second = run;
            }
            last = run;

            run = run.right;
            while(run != null){
                stack.add(run);
                run = run.left;
            }
        }
        int swap = first.val;
        first.val = second.val;
        second.val = swap;
    }
}