# Write a Python function to find the single element in a list where every element appears three times except for one.

## Topic: Write a Python function to find the single element in a list where every element appears three times except for one.

Solution

def single_number(arr): ones, twos = 0, 0 for x in arr: ones, twos = (ones ^ x) & ~twos, (ones & x) | (twos & ~x) assert twos == 0 return ones

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