DSA interview question

Check Whether the K-th Bit Is Set

Easy · Algorithms · Topics: bit manipulation

The problem

A permissions system stores each user's rights as bits of one integer: bit 0 = read, bit 1 = write, bit 2 = delete, and so on. Before an action runs, the server must answer "is bit k of this user's permission mask set?" millions of times a second. The answer is a single shift and mask: move bit k down to position 0 and check it, or build a mask with only bit k set and AND it with the number.

Example: n = 4 (binary 100), k = 2 → true; k = 0 → false.

Function signature

bool isKthBitSet(int n, int k);  // k = 0 is the least significant bit, 0 <= k <= 31

Test cases

Input `(n, k)`	Output	Explanation
`(4, 0)`	`false`	4 = 100
`(4, 2)`	`true`
`(500, 3)`	`false`	500 = 111110100
`(500, 4)`	`true`
`(-1, 31)`	`true`	The sign bit of −1 is set

(Confirm: bits are numbered from 0 at the least significant end; n may be negative, in which case its two's complement bits are checked.)

How interviewers phrase it

"Given a number n and a bit number k, check if the k-th bit of n is set" (GeeksforGeeks).

Clarifying questions to ask

Is k 0-indexed or 1-indexed, and from which end?
Can n be negative?
What's the range of k? (0 to 31 for a 32-bit int.)

Follow-ups to expect

"Right shift or left shift?"
"What goes wrong with 1 << 31?"
"How do you set, clear and toggle bit k?"
"Why is this O(1)?"
"How do you check several bits at once?"

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