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24. Reverse Bits

easyAsked at Salesforce

Reverse the bits of a given 32-bit unsigned integer. Salesforce uses this to gauge whether candidates can comfortably reason about bit manipulation.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Salesforce loops.

  • Glassdoor (2026-Q1)— Asked occasionally on Salesforce platform-team phone screens.
  • LeetCode Discuss (2025)— Used as a low-level bit-manip filter.

Problem

Reverse bits of a given 32 bits unsigned integer.

Constraints

  • The input must be a binary string of length 32

Examples

Example 1

Input
n = 00000010100101000001111010011100
Output
964176192 (00111001011110000010100101000000)

Example 2

Input
n = 11111111111111111111111111111101
Output
3221225471 (10111111111111111111111111111111)

Approaches

1. Convert to binary string and reverse

Convert to 32-char binary string (pad with zeros), reverse, parse back.

Time
O(32)
Space
O(32)
function reverseBits(n) {
  const bin = n.toString(2).padStart(32, '0');
  return parseInt([...bin].reverse().join(''), 2);
}

Tradeoff: Works but uses string allocation. Salesforce prefers the bit-shift version.

2. Bit-shift loop

Loop 32 times: extract LSB of n, shift result left, OR in the bit, shift n right.

Time
O(32)
Space
O(1)
function reverseBits(n) {
  let result = 0;
  for (let i = 0; i < 32; i++) {
    result = (result << 1) | (n & 1);
    n >>>= 1;
  }
  return result >>> 0;
}

Tradeoff: Pure bitwise. The >>> 0 at the end coerces to unsigned 32-bit, which JavaScript needs because bitwise ops use signed 32-bit.

Salesforce-specific tips

Salesforce platform-team uses bit manipulation in their permission masks and field-level security tracking. They grade on the >>> 0 coercion — forgetting it returns a negative number in JS. Bonus signal: mention the bit-block reversal trick (divide-and-conquer that reverses in O(log 32) operations).

Common mistakes

  • Using >> instead of >>> — preserves sign bit, giving negative results.
  • Forgetting the final >>> 0 — JavaScript bitwise ops return signed int32.
  • Reversing only the bits used by n.toString(2) — loses leading zeros.

Follow-up questions

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

  • Reverse bits in O(log n) using divide-and-conquer.
  • Number of 1 Bits (LC 191).
  • Counting bits 0..n (LC 338).

Solve it now

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Output

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FAQ

Why do we need >>> 0 at the end?

JavaScript bitwise ops use signed 32-bit. If the top bit of result is 1, it would be interpreted as a negative number. >>> 0 coerces to unsigned.

Is there a faster approach for many calls?

Yes — precompute reversals for each byte and combine. Reduces 32 iterations to 4 lookups. Mention as a real-world optimization.

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