After you see the algo you will think, wow that was quite easy :D

Now do it without loop, in O(1) ;)

No random tests, no one approve it.

That is wrong.

That's because how type promotions work in C.

This will not work: long toRet = firstNumber + secondNumber because result of addition of two ints is also an int, so addition overflows, and then such overflowed result is assigned to toRet. Addition result is not extended to long until actual assignment.

However, these two will work:

long toRet = (long)firstNumber + secondNumber, and

return (long)firstNumber + secondNumber

They work because you explicitly promote one of operands to long, so actual addition is performed with longs and does not overflow.

Main difference is: in the first case, you work with ints and you add two ints, receiving int in the result. In two remaining cases, you cast one operand to long, the other gets implicitly promoted to long, so you perform addition on two longs.

Description updated

Explain