##### Short Problem Definition:

Given a table A of N integers from 0 to N-1 calculate the smallest such index P, that that {A[0],…,A[N-1]} = {A[0],…,A[P]}.

##### Link

##### Complexity:

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

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

##### Execution:

Based on the number of elements that can occur in N, you either mark the occurrences in a boolean array, or put them in a set. The last occurrence of an element that was not seen before is the result.

##### Solution:

```
def solution(A):
seen = set()
smallest_prefix_idx = 0
for idx in xrange(len(A)):
if A[idx] not in seen:
seen.add(A[idx])
smallest_prefix_idx = idx
return smallest_prefix_idx
```