By making use of recursion, we can multiply two integers with the given constraints.
To multiply x and y, recursively add x y times.
int multiply(int x, int y){ /* 0 multiplied with anything gives 0 */ if(y == 0) return 0; /* Add x one by one */ if(y > 0 ) return (x + multiply(x, y-1)); /* the case where y is negative */ if(y < 0 ) return -multiply(x, -y);}