What is the test case which is not covered in the code written by me

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https://www.codechef.com/viewsolution/25014193

the above is a link to a solution of the BST problem. I am yet to start my second year and i am new to DS, using the concepts this is my first implementation. I can’t figure out why the CodeChef evaluator is giving ‘wrong answer’ every time. Is it any test case which i have forgotten to incorporate or some constrain condition which i am missing out? thank you.