1ms recursive Java Solution, easy to understand
 public boolean isSymmetric(TreeNode root) {\n        if(root==null) return true;\n        return isMirror(root.left,root.right);\n    }\n    public boolean isM
     public boolean isSymmetric(TreeNode root) {\n        if(root==null) return true;\n        return isMirror(root.left,root.right);\n    }\n    public boolean isMirror(TreeNode p, TreeNode q) {\n        if(p==null && q==null) return true;\n        if(p==null || q==null) return false;\n        return (p.val==q.val) && isMirror(p.left,q.right) && isMirror(p.right,q.left);\n    }
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