Leetcode invert binary tree

tree.md

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+# Binary tree
+
++ [Invert Binary Tree](#invert-binary-tree)
+
+## Invert Binary Tree
+
+https://leetcode.com/problems/invert-binary-tree/
+
+```python
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution(object):
+    def invertTree(self, root):
+        if root:
+            root.left, root.right = self.invertTree(root.right), self.invertTree(root.left)
+            return root
+
+```