https://codility.com/demo/results/trainingA9U3W3-44N/ yikes

MissingInteger.py

# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"

def solution(A):
    # write your code in Python 2.7
    N = len(A)
    C = [0] * N
    res = 0
    
    for i in A:
        if i > 0 and i <= N:
            C[i-1] += 1
    
    for i in range(1, N+1):
        if not C[i-1]:
            res = i
            break
    
    if not res: res = N + 1
    
    return res