#145 Easy Time O(1) Space O(1) Animated

Number of 1 Bits

Return the number of set bits (Hamming weight) in n.

Tutor Mode Build the mental model before you memorize the code.

Pattern Bitwise state reduction

Use a bit operation to retain only the information the problem needs while discarding the rest.

Approach: Bit Manipulation (n & n-1)

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Core Insight

Use a bit operation to retain only the information the problem needs while discarding the rest.

Common Pitfall

Parenthesize shifts and masks, especially when mixing comparison or arithmetic operators.

Description

Return the number of set bits (Hamming weight) in n.

TimeO(1) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

def hammingWeight(n):
    res = 0
    while n:
        n &= (n - 1)
        res += 1
    return res