101. Symmetric Tree

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def isSymmetric(self, root: TreeNode) -> bool:
        if not root:
            return True
        else:
            return self.checkSymmetric(root.left, root.right)
    
    def checkSymmetric(self, left: TreeNode, right: TreeNode):
        if not left and not right:
            # symmetric terminal nodes
            return True
        elif (not left and right) or (left and not right):
            # one node terminal when other isn't
            return False
        elif left.val == right.val:
            # check further
            outside = self.checkSymmetric(left.left, right.right)
            inside = self.checkSymmetric(left.right, right.left)
            return outside and inside
        else:
            # not symmetric
            return False