 python program to check for symmetric binary tree without using recursion

DescriptionWe use a Queue here. For a symmetric that elements at every level are palindromic.
In other words,
1. The left child of left subtree = right child of right subtree.
2. The right child of left subtree = left a child of right subtree.
If we insert the left child of left subtree first followed by the right child of the right subtree in the queue, we only need to ensure that these are equal.
Similarly, If we insert the right child of left subtree followed by the left child of the right subtree in the queue, we again need to ensure that these are equal.

Code:
# Python3 program to program to check if a # given Binary Tree is symmetric or not # Helper function that allocates a new # node with the given data and None # left and right pairs. class newNode: # Constructor to create a new node def __init__(self, key): self.key = key self.left = None self.right = None # function to check if a given # Binary Tree is symmetric or not def isSymmetric( root) : # if tree is empty if (root == None) : return True # If it is a single tree node, # then it is a symmetric tree. if(not root.left and not root.right): return True q = [] # Add root to queue two times so that # it can be checked if either one # child alone is NULL or not. q.append(root) q.append(root) # To store two nodes for checking # their symmetry. leftNode = 0 rightNode = 0 while(not len(q)): # Remove first two nodes to # check their symmetry. leftNode = q q.pop(0) rightNode = q q.pop(0) # if both left and right nodes # exist, but have different # values-. inequality, return False if(leftNode.key != rightNode.key): return False # append left child of left subtree # node and right child of right # subtree node in queue. if(leftNode.left and rightNode.right) : q.append(leftNode.left) q.append(rightNode.right) # If only one child is present # alone and other is NULL, then # tree is not symmetric. elif (leftNode.left or rightNode.right) : return False # append right child of left subtree # node and left child of right subtree # node in queue. if(leftNode.right and rightNode.left): q.append(leftNode.right) q.append(rightNode.left) # If only one child is present # alone and other is NULL, then # tree is not symmetric. elif(leftNode.right or rightNode.left): return False return True # Driver Code if __name__ == '__main__': # Let us construct the Tree # shown in the above figure root = newNode(1) root.left = newNode(2) root.right = newNode(2) root.left.left = newNode(3) root.left.right = newNode(4) root.right.left = newNode(4) root.right.right = newNode(3) if (isSymmetric(root)) : print("The given tree is Symmetric") else: print("The given tree is not Symmetric")

Output:
The given tree is Symmetric 