Symmetric Binary Tree

https://leetcode.com/problems/symmetric-tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3
 

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3
 

Note: Bonus points if you could solve it both recursively and iteratively.

Implementation :

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null)
            return true;
        return isSymmetric(root.left, root.right);
    }
    
    public boolean isSymmetric(TreeNode left, TreeNode right){
        if(left == null && right == null)
            return true;
        else if (left == null || right == null)
            return false;
        else if (left.val != right.val)
            return false;
        return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
    }
}

References :

1. https://www.youtube.com/watch?v=K7LyJTWr2yA
2. https://www.youtube.com/watch?v=XV7Sg2hJO3Q