##### Short Problem Definition:

There are *N* integers in an array *A*. All but one integer occur in pairs. Your task is to find out the number that occurs only once.

##### Link

##### Complexity:

time complexity is O(N);

space complexity is O(1)

##### Execution:

XORing two equal numbers cancels them out. XOR all numbers together.

##### Solution:

#!/usr/bin/py def lonelyinteger(a): answer = 0 for candidate in a: answer ^= candidate return answer if __name__ == '__main__': a = input() b = map(int, raw_input().strip().split(" ")) print lonelyinteger(b)

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