【LEETCODE】226-Invert Binary Tree

Invert a binary tree.
     4
  / \
  2     7
 /\ /\
1   3 6   9
to
     4
  / \
  7     2
 /\ /\
9   6 3   1

# 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 invertTree(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if root == None:                                      #root  ，    
            return root
        elif root.left == None and root.right == None:        #  root，    
            return root
        else:
            tmp = root.left                                   #    child
            root.left = root.right
            root.right = tmp
            
            root.left = self.invertTree(root.left)            #      child
            root.right = self.invertTree(root.right)
            
            return root