25. Reverse Bits

easyAsked at Workday

Reverse the bits of a 32-bit unsigned integer. Workday uses this to test bitwise fluency — needed when packing role/permission flags into compact bitmaps for RBAC.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Source citations

Public interview reports confirming this problem appears in Workday loops.

Glassdoor (2025)— Workday RBAC team — bitwise warmup.

Problem

Reverse bits of a given 32 bits unsigned integer.

Constraints

The input must be a binary string of length 32.
Follow-up: If this function is called many times, how would you optimize it?

Examples

Example 1

Input

n = 00000010100101000001111010011100

Output

964176192 (00111001011110000010100101000000)

Example 2

Input

n = 11111111111111111111111111111101

Output

3221225471 (10111111111111111111111111111111)

Approaches

1. String reverse

Convert to binary string, pad to 32, reverse, parse.

Time: O(32)
Space: O(32)

return parseInt(n.toString(2).padStart(32,'0').split('').reverse().join(''), 2);

Tradeoff: Works but slow and string-heavy. Demo only.

2. Bit shift accumulate

For 32 iterations: shift result left, OR in n's lowest bit, shift n right.

Time: O(32) = O(1)
Space: O(1)

function reverseBits(n) {
  let result = 0;
  for (let i = 0; i < 32; i++) {
    result = (result << 1) | (n & 1);
    n = n >>> 1;
  }
  return result >>> 0; // force unsigned
}

Tradeoff: Constant time. The >>> 0 at the end forces JS to interpret as unsigned 32-bit.

Workday-specific tips

Workday grades the >>> 0 unsigned-cast at the end — JS bitwise ops are signed 32-bit by default, so without it you can return a negative number on inputs with the top bit set. Mention the RBAC analogy.

Common mistakes

Using >> instead of >>> for n — leaks sign bit into the high bits.
Forgetting >>> 0 at the end — returns a signed value.
Looping a variable number of times instead of exactly 32 — leading zeroes are part of the input.

Follow-up questions

An interviewer at Workday may pivot to one of these next:

Cache 8-bit reversal in a lookup table — process 4 bytes per call.
Number of 1 Bits (LC 191).
Bitwise AND of Numbers Range (LC 201).

Solve it now

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Output

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FAQ

Why >>> instead of >>?

>>> is unsigned (logical) right shift — fills with 0s. >> is arithmetic — fills with the sign bit. For 32-bit reversal we want pure logical.

How would I optimize for many calls?

Precompute an 8-bit reversal table (256 entries). Then reverse 4 bytes per call with three lookups and shifts. Constant time but cache-friendly.

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