Python given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A in O(n) time complexity

Testing for the presence of a number in a set is fast in Python so you could try something like this:

def minpositive(a):
    A = set(a)
    ans = 1
    while ans in A:
       ans += 1
    return ans

def solution(A):
    B = set(sorted(A))
    m = 1
    for x in B:
        if x == m:
            m+=1
    return m