please help me how to wirte it in python or C++

A bit string is either lambda (the empty string) or it has the form omega O or omega 1 for some bit string Omega. Using the above recursive definition, give a recursive algorithm for finding the number of 1's in a bit string. Write your algorithm as a procedure in the style of your textbook or as a function in some programming language. Name the algorithm count_ones. Thus, for example, (10010) should return 2 and count_ones(00) should return 0