##### Short Problem Definition:

Find the missing element in a given permutation.

##### Link

##### Complexity:

expected worst-case time complexity is `O(N)`

expected worst-case space complexity is `O(1)`

##### Execution:

Sum all elements that should be in the list and sum all elements that actually are in the list. The sum is 0 based, so +1 is required. The first solution using the + operator can cause int overflow in not-python languages. Therefore the use of a binary XOR is adequate.

##### Solution:

```
def solution(A):
should_be = len(A) # you never see N+1 in the iteration
sum_is = 0
for idx in xrange(len(A)):
sum_is += A[idx]
should_be += idx+1
return should_be - sum_is +1
```

```
def solution(A):
missing_element = len(A)+1
for idx,value in enumerate(A):
missing_element = missing_element ^ value ^ (idx+1)
return missing_element
```