The Odd One Among Triples

Medium

A sensor sends readings, and each value normally arrives exactly three times, since the signal is tripled for reliability. Only one value arrived a single time. Find it. The readings come in any order. A dictionary solution is easy; try for linear time and O(1) extra memory.

Example 1

readings=[4, -1, 4, 4] → -1

4 shows up three times, -1 only once.

Example 2

readings=[7, 0, 5, 0, 7, 5, 7, 0, 5, 12] → 12

7, 0 and 5 each appear three times; 12 appears once.

Example 3

readings=[-30] → -30

Constraints

1 ≤ len(readings) ≤ 3·10⁴
-2³¹ ≤ readings[i] ≤ 2³¹ - 1
Exactly one value appears once; every other value appears exactly three times

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Topics

Bit Manipulation Array

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Input

readings=[4, -1, 4, 4]

Expected

-1

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