algorithm - Finding a single number in a list

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/35185/finding-a-single-number-in-a-list

Answer 1

The fastest (O(n)) and most memory efficient (O(1)) way is with the XOR operation.

In C:

int arr[] = {3, 2, 5, 2, 1, 5, 3};

int num = 0, i;

for (i=0; i < 7; i++)
    num ^= arr[i];

printf("%i
", num);

This prints "1", which is the only one that occurs once.

This works because the first time you hit a number it marks the num variable with itself, and the second time it unmarks num with itself (more or less). The only one that remains unmarked is your non-duplicate.