DSA interview question

Check Whether a Number Is Odd Using Bits

Easy · Algorithms · Topics: bit manipulation

The problem

A load balancer alternates requests between two server pools based on whether the request ID is odd or even. The check runs on every request and must be correct for negative IDs too, which some legacy clients send. n % 2 == 1 is a classic trap: in C++, -3 % 2 is -1, so negative odd numbers are misclassified. Looking at the least significant bit avoids both the division and the sign problem: in two's complement, a number is odd exactly when bit 0 is 1.

Example: 7 → odd; −4 → even.

Function signature

bool isOdd(int n);

Test cases

Input	Output	Explanation
`7`	`odd`	111 ends in 1
`10`	`even`	1010 ends in 0
`0`	`even`
`-3`	`odd`	Two's complement ...1101 ends in 1, even though −3 % 2 is −1
`-4`	`even`	...1100

(Confirm: n is a 32-bit signed integer, possibly negative.)

How interviewers phrase it

"Check whether a number is even or odd using bit manipulation" (GeeksforGeeks).

Clarifying questions to ask

Can n be negative?
Must I avoid % and /? (That's the point of the exercise.)
What should I return: a bool or the strings "odd"/"even"?

Follow-ups to expect

"Why does n & 1 work?"
"Why is n % 2 == 1 buggy?"
"How do you check divisibility by 2^k?"
"Is n & 1 really faster?"
"Complexity?"

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