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?

This is a very important skill while traversing tree.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    TreeNode prev = null;
    TreeNode first = null;
    TreeNode second = null;
    public void recoverTree(TreeNode root) {

        findSwap(root);

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

    void findSwap(TreeNode root){
        if(root == null) return;

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

        prev = root;

        findSwap(root.right);
    }
}