递归
def isSymmetric(self, root: TreeNode) -> bool:
def st(A, B):
if A == B == None:
return True
if not A or not B:
return False
return A.val==B.val and st(A.left, B.right) and st(A.right, B.left)
if not root:
return True
return st(root.left, root.right)
非递归
def isSymmetric(self, root: TreeNode) -> bool:
if not root:
return True
stack = [(root.left, root.right)]
while stack:
l, r = stack.pop()
if l == r == None:
continue
if not l or not r or l.val != r.val:
return False
stack.append((l.left, r.right))
stack.append((l.right, r.left))
return True
