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24. Reverse Bits

easyAsked at Plaid

Reverse the bits of a 32-bit unsigned integer. Plaid asks this as a bitwise-fluency check before harder hash-collision and idempotency-key problems.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • Glassdoor (2025)— Plaid systems screen.
  • LeetCode Discuss (2026)— Plaid OA — bit-manipulation warm-up.

Problem

Reverse the bits of a given 32-bit unsigned integer. Note: In JavaScript, integers are 32-bit signed under bitwise ops; use unsigned right shift (>>> 0) to interpret the result as unsigned.

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. String reversal

Convert to a 32-char binary string, reverse, parse back.

Time
O(32)
Space
O(32)
function reverseBits(n) {
  return parseInt(n.toString(2).padStart(32, '0').split('').reverse().join(''), 2);
}

Tradeoff: Works but dodges the bit math. Plaid wants to see the shift loop.

2. Bit-by-bit shift

Pop the LSB of n with (n & 1), push it into result. Shift n right, shift result left. 32 iterations.

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

Tradeoff: Constant time (32 bits), constant space. The >>> 0 at the end coerces to unsigned for the return value.

Plaid-specific tips

Plaid grades this on whether the shift loop comes naturally. Bonus signal: mention the constant-time mask-and-swap optimization (swap halves, then quarters, etc., in log2(32) = 5 steps). Connect bit reversal to hashing for idempotency keys.

Common mistakes

  • Using >> (signed) instead of >>> (unsigned) — corrupts the high bit.
  • Forgetting the final >>> 0 — returns a negative number in JS.
  • Looping only until n becomes 0 — drops leading zeros, getting the wrong reversed value.

Follow-up questions

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

  • Mask-and-swap in 5 steps (constant-time, no loop).
  • Reverse only the lower k bits.
  • Count set bits (LC 191).

Solve it now

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Output

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FAQ

Why >>> instead of >> in JS?

>> sign-extends — the high bit gets sticky. >>> is logical right shift, zero-filled. For unsigned bit ops, >>> is the safe default.

What's the log-steps optimization?

Swap each pair of bits, then each pair of pairs (4-bit blocks), then 8-bit, 16-bit. Five steps total instead of 32. Useful when called millions of times.

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