24. Reverse Bits

easyAsked at Snowflake

Reverse the bits of a 32-bit unsigned integer. Snowflake asks this to test bit-manipulation fluency — relevant for compact column encodings where they sometimes pack rotated/reversed bits to align with vectorized SIMD masks.

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

Source citations

Public interview reports confirming this problem appears in Snowflake loops.

Glassdoor (2025-Q4)— Snowflake storage-team uses this in onsites for low-level encoding discussions.
LeetCode Discuss (2025-07)— Asked at Snowflake intern screens.

Problem

Reverse bits of a given 32 bits unsigned integer. In JavaScript, the function will only be given as input a signed integer type. They should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned.

Constraints

The input must be a binary string of length 32.

Examples

Example 1

Input

n = 00000010100101000001111010011100

Output

964176192 (00111001011110000010100101000000)

Approaches

1. Convert to binary string, reverse, parse

Convert to 32-bit padded binary string; reverse; parse as int.

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

function reverseBits(n) {
  const bits = n.toString(2).padStart(32, '0');
  const reversed = bits.split('').reverse().join('');
  return parseInt(reversed, 2) >>> 0;
}

Tradeoff: Allocates strings. Works but defeats the purpose.

2. Bit-by-bit shift (optimal for interview)

32 iterations: extract LSB of n, shift left into result.

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;
}

Tradeoff: Pure bit ops. The >>> 0 at the end forces unsigned interpretation in JS.

Snowflake-specific tips

Snowflake interviewers want the bit-by-bit version and to hear you discuss the >>> vs >> distinction in JS. Bonus signal: discuss byte-swap intrinsics (e.g., bswap, vrev) used in their SIMD vectorized kernels for byte-order-sensitive encodings.

Common mistakes

Using >> instead of >>> — propagates sign bit, breaks for negative inputs.
Forgetting that JS bitwise ops are 32-bit even though Number is 64-bit double.
Hardcoding loop bound to 30 or 31 instead of 32.

Follow-up questions

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

Reverse using a divide-and-conquer mask trick (swap halves, then quarters, then bytes, then nibbles, then pairs).
Reverse bits of a 64-bit number.
How does Snowflake's columnar encoding use bit-level packing?

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Output

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FAQ

Why >>> 0 at the end?

JS's signed-bit interpretation can give negative numbers for high bits. >>> 0 forces unsigned 32-bit interpretation so the return value matches LC's expected format.

Is there a faster way?

Yes — the divide-and-conquer 'swap halves, then quarters, then bytes...' approach does it in log(32) = 5 steps. Worth mentioning but bit-by-bit is what most interviewers expect.

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