##### Short Problem Definition:

Find the minimal positive integer not occurring in a given sequence.

##### Link

##### Complexity:

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

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

##### Execution:

You only need to consider the first (N) positive integers. In this specification 0 does not count as a valid candidate! Any value that is below 1 or above N can be ignored.

##### Solution:

```
def solution(A):
seen = [False] * len(A)
for value in A:
if 0 < value <= len(A):
seen[value-1] = True
for idx in xrange(len(seen)):
if seen[idx] == False:
return idx + 1
return len(A)+1
```