Symmetric Tree

Java

class Solution {
	public boolean isSymmetric(TreeNode root) {
		if (root == null) return true;
		return helper(root.left, root.right);
	}
	private boolean helper(TreeNode left, TreeNode right) {
		if (left == null && right == null) return true;
		if (left == null || right == null) return false;
		
		return left.val == right.val && helper(left.left, right.right) && helper(left.right, right.left);
	}
}