#144 Easy Time O(n) Space O(1) Animated

Single Number

Find the element that appears once when others appear twice.

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: Bitwise XOR

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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

Find the element that appears once when others appear twice.

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

def singleNumber(nums):
    res = 0
    for n in nums:
        res = n ^ res
    return res