Question

how to write a algorithm for sum of digits

Answer 1

Well you'll have to take two variables of type int(say)

int x;
int y;

x=x+y

print(x);

Answer 2

Are you asking about a function, that would take a number like this 111 and return the sum of the digits of this number, in this case 3?
There is a course right now on edx.org that covers this topic in the first week in two ways. It is called "Louv1.01x Paradigms of Computer Programming". If you would like to have a nice video about this kind of algorithm, look there. The core point will be that you have to use integer division and a modulo function.

Answer 3

public int sumDigits(int num) {
int sum = 0;

while (num != 0){
sum += num %10;
num = num /10;
}
return sum;
}