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# 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.
        """
        self.node1 = None
        self.node2 = None
        self.prev = None
        self.DFS(root)
        temp = self.node1.val
        self.node1.val = self.node2.val
        self.node2.val = temp
    def DFS(self, curr):
        if curr == None:
            return
        self.DFS(curr.left)
        if self.prev != None:
            # At the very beginning, self.prev = None.
            if self.prev.val > curr.val:
                # Anormaly detected.
                if self.node1 == None:
                    # There could be 2 anoramlies, e.g. [1,2,7,4,5,6,3,8,9]
                    # The `3` and `7` are swapped, so (7,4) and (6,3) are anormalies.
                    self.node1 = self.prev
                self.node2 = curr
        self.prev = curr
        self.DFS(curr.right)