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.

Solution

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

    public boolean helper(TreeNode node1, TreeNode node2) {
        if (node1 == null && node2 == null) {
            return true;
        } else if (node1 == null || node2 == null) {
            return false;
        } else {
            return node1.val == node2.val && helper(node1.left, node2.right) && helper(node1.right, node2.left);
        }
    }
}