190. Reverse Bits

At a Glance

• Topic: bit-manipulation
• Pattern: Bit ops / shift accumulate (Bit Manipulation)
• Difficulty: Easy
• Companies: Amazon, Apple, Microsoft, Google, Adobe
• Frequency: High
• LeetCode: 190

Problem (one-liner)

Reverse the bit order of a 32-bit unsigned integer and return the value (LeetCode treats as unsigned for this problem).

Recognition Cues

• "Reverse bits" of fixed width
• Build result by shifting in bits from LSB to MSB of output

Diagram

At-a-glance flow (replace with problem-specific Mermaid as you refine this note). camelCase node IDs; no spaces in IDs.

Approaches

• Brute force — stringify bits — O(32) with extra space.
• Optimal — iterative bit extraction and push — O(32) time / O(1) space.

Optimal Solution

Go

package main

func reverseBits(value uint32) uint32 {
	var result uint32 = 0
	for shift := 0; shift < 32; shift++ {
		result <<= 1
		result |= value & 1
		value >>= 1
	}
	return result
}

JavaScript

function reverseBits(value) {
	let result = 0;
	let remaining = value >>> 0;
	for (let shift = 0; shift < 32; shift++) {
		result <<= 1;
		result |= remaining & 1;
		remaining >>>= 1;
	}
	return result >>> 0;
}

Walkthrough

Small pattern on 8 bits for intuition — mirror each incoming least significant bit into growing result.

Invariant: after shift steps, result holds the reversed lower shift bits of the original in its upper construction.

Edge Cases

• All zeros / all ones
• Palindrome bit patterns

Pitfalls

• Forgetting exactly 32 iterations
• JS signed vs unsigned — use >>> / >>> 0 return

Similar Problems

• 191. Number of 1 Bits — traverses set bits differently.
• 007. Reverse Integer — decimal reverse (canonical math topic).

Variants

• Reverse within subrange of bits.
• Swap endianness for specific sizes (16/64).

Mind-Map Tags

#reverse-bits #shift #unsigned #fixed-width #bit-ops