##### 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)
```