PermMissingElem

Link: https://app.codility.com/programmers/lessons/3-time_complexity/perm_missing_elem/

An array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing.

Your goal is to find that missing element.

Write a function:

class Solution { public int solution(int[] A); }

that, given an array A, returns the value of the missing element.

For example, given array A such that:

A[0] = 2
A[1] = 3
A[2] = 1
A[3] = 5
the function should return 4, as it is the missing element.

Write an efficient algorithm for the following assumptions:

N is an integer within the range [0..100,000];
the elements of A are all distinct;
each element of array A is an integer within the range [1..(N + 1)].

Tried

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14

from collections import Counter

def solution(A):
    try:
        count_A = Counter(A)
        for i in range(1, max(A)):
            if i not in count_A.keys():
                return i
        raise Exception("not found")
    except:
        if len(A) > 0:
            return max(A)+1
        else:
            return 1

1,2,3 일 때는 그럼 어떡하지 했는데, 4인 걸 제출하고나서야 알았다.
그래서 위처럼 한번 수정했더니 100% 만족함을 알았다.