Iterative — Easy : https://leetcode.com/problems/symmetric-tree/
Recursive — Medium [ very important and intuitive]
Hint : 1. A Tree is symmetric to itself. So if you implement a function which accepts two trees to check for symmetry, you can start the first call of the function with symmetric(root, root).
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).