99.py

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

	Recover the tree without changing its structure.

	Example 1:

	Input: [1,3,null,null,2]

	   1
	  /
	 3
	  \
	   2

	Output: [3,1,null,null,2]

	   3
	  /
	 1
	  \
	   2
'''
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def recoverTree(self, root):
        """
        :type root: TreeNode
        :rtype: void Do not return anything, modify root in-place instead.
        """

        first, second, prev = None, None, None
        def inorder(root):
        	if root:
        		inorder(root.left)
        		if prev is not None and root.val < prev.val:
        			if first is None:
        				first = root
        			else:
        				second = root
        		prev = root
        		inorder(root.right)


        inorder(root)
        if first and second:
        	first.val, second.val = second.val, first.val