Clean Java solution
public class Solution {\n        public boolean isSymmetric(TreeNode root) {\n            return isSymmetric(root, root);\n        }\n        \n        boolean 
    public class Solution {\n        public boolean isSymmetric(TreeNode root) {\n            return isSymmetric(root, root);\n        }\n        \n        boolean isSymmetric(TreeNode n1, TreeNode n2) {\n            if(n1 == null && n2 == null) return true;\n            if(n1 == null || n2 == null) return false;\n            if(n1.val != n2.val) return false;\n            return isSymmetric(n1.left, n2.right) && isSymmetric(n2.right, n1.left);\n        }\n    }
['Java']