Facts and thought process.
If a > b, then there are no numbers that are greater than or equal to a and less than or equal to b. The sum of no numbers is 0.
If a <= b, then
sum(a,b) | = a + (a+1) + ... + b |
= a + sum(a+1, b) |
C++ definition of sum
int sum(int a, int b) { if(a > b) { return 0; } else { return a + sum(a+1, b); } }