LC 0101 [E] Symmetric Tree - ALawliet/algorithms GitHub Wiki

T: O(b)
S: O(logn)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isSymmetric(self, root):
        def isMirror(left, right):
            if not left and not right: # no subtrees
                return True
            
            if not left or not right: # one subtree is missing
                return False

            if left.val == right.val:
                outer = isMirror(left.left, right.right)
                inner = isMirror(left.right, right.left)
                return outer and inner
            else: # different values
                return False
        
        if not root:
            return True
        else: # check subtrees
            return isMirror(root.left, root.right)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isSymmetric(self, root):
          if not root:
              return True
        
          stack = [ (root.left, root.right) ]
          while stack:
              left, right = stack.pop()
                
              if not left and not right:
                  continue
                    
              if not left or not right:
                  return False
                
              if left.val == right.val:
                  stack.append( (left.left, right.right) )
                  stack.append( (left.right, right.left) )
              else:
                  return False
                
          return True