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Add solution for the "Number of 1 Bits" problem with unit tests. #46

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Merged
kobukuro merged 1 commit into from
Feb 23, 2026
Merged

Add solution for the "Number of 1 Bits" problem with unit tests. #46

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19 changes: 19 additions & 0 deletions src/main/java/com/leetcode/easy/NumberOf1Bits191.java
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Original file line number Diff line number Diff line change
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// Tags: Bit Manipulation, Divide and Conquer
package com.leetcode.easy;

public class NumberOf1Bits191 {
/**
* Time complexity: O(1)
* Space complexity: O(1)
*/
public int hammingWeight(int n) {
int res = 0;
while (n != 0) {
if (n % 2 == 1) {
res++;
}
n = n >> 1;
Comment on lines +12 to +15
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Copilot AI Feb 22, 2026

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hammingWeight uses n % 2 == 1 and an arithmetic right shift (n >> 1). For negative inputs (which occur when interpreting the LeetCode "unsigned 32-bit" value as a Java int), n % 2 is negative and n >> 1 preserves the sign bit, so n may never reach 0 (e.g., n = -1 loops forever) and the bit count is incorrect. Use bit operations that work for all 32 bits, e.g. check (n & 1) != 0 and shift with n >>>= 1, or use the common n &= (n - 1) loop.

Suggested change
if (n % 2 == 1) {
res++;
}
n = n >> 1;
if ((n & 1) != 0) {
res++;
}
n >>>= 1;

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}
return res;
}
}
39 changes: 39 additions & 0 deletions src/test/java/com/leetcode/easy/NumberOf1Bits191Test.java
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package com.leetcode.easy;

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

public class NumberOf1Bits191Test {
private NumberOf1Bits191 solution;

@BeforeEach
void setUp() {
solution = new NumberOf1Bits191();
}

@Test
void testHammingWeight_Example1() {
int n = 11; // Binary representation: 1011
int expected = 3;
int result = solution.hammingWeight(n);
assertEquals(expected, result);
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Copilot AI Feb 22, 2026

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These tests only cover positive int inputs, but this problem treats the argument as an unsigned 32-bit value; in Java that includes negative ints (e.g. n = -1 should return 32). Adding a negative-input test (and optionally n = 0) would match the problem contract and catch issues with signed shifting/modulus in the implementation.

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}

@Test
void testHammingWeight_Example2() {
int n = 128; // Binary representation: 10000000
int expected = 1;
int result = solution.hammingWeight(n);
assertEquals(expected, result);
}

@Test
void testHammingWeight_Example3() {
int n = 2147483645; // Binary representation: 1111111111111111111111111111101
int expected = 30;
int result = solution.hammingWeight(n);
assertEquals(expected, result);
}
}