It’s the most curious and excellent problem.
at first i thought to form a brute-force one which could have taken
which i trash it as usually. But now i’m feeling that brute-force one would be more difficult to code than this efficient and easy one!!!!
we have to use a dummy null tree in the global space to overcome both the const return compilation error and local var return compilation error
PS: error handling is ignored i.e if b is in the path or if a or b or both are absent in the tree
PPS: we just have to embed an if condition and make the end product to be returned as NULL from the d_c_n for all errors