Create 101_Symmetric_Tree.md

Author: koji

101_Symmetric_Tree.md

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+## 101. Symmetric Tree
+
+```
+Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
+
+For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
+
+ 1
+ / \
+ 2 2
+ / \ / \
+3 4 4 3
+ 
+
+But the following [1,2,2,null,3,null,3] is not:
+
+ 1
+ / \
+ 2 2
+ \ \
+ 3 3
+ 
+
+Follow up: Solve it both recursively and iteratively.
+```
+
+```python
+def isSymmetric(self, root: TreeNode) -> bool:
+ if not root: return True
+ return self.isMirror(root.left, root.right)
+ 
+ def isMirror(self, left, right):
+ if not left and not right: return True
+ if not left or not right: return False
+ if left.val == right.val:
+ return self.isMirror(left.left, right.right) and self.isMirror(left.right, right.left)
+ else:
+ return False
+
+```
+
+```
+Runtime: 32 ms, faster than 81.08% of Python3 online submissions for Symmetric Tree.
+Memory Usage: 14.1 MB, less than 100.00% of Python3 online submissions for Symmetric Tree.
+```