Sharing an answer code of mine about MissingInteger problem of Codility lesson 4.

Lesson 4: MissingInteger

This is a demo task.

Write a function:

def solution(A, K)

that, given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.

For example, given A = [1, 3, 6, 4, 1, 2], the function should return 5.

Given A = [1, 2, 3], the function should return 4.

Given A = [−1, −3], the function should return 1.

Assume that:

  • N is an integer within the range [1..100,000];
  • each element of array A is an integer within the range [−1,000,000..1,000,000].


  • expected worst-case time complexity is O(N);
  • expected worst-case space complexity is O(N), beyond input storage (not counting the storage required for input arguments).

Elements of input arrays can be modified.

Example answer code in Python 2.7

def solution(A):
    # write your code in Python 2.7
    We can only consider N integers as we are tying to find the smallest positive integer.    
    occur = [False]*len(A)
    for value in A: # Save occured positive integer less than N
        if 0 < value <= len(A):
            occur[value-1] = True
    for i in range(len(occur)): # Check the smallest positive integer
        if occur[i] == False:
            return i+1
    return len(occur)+1 # All positive integer less than N exist, so next integer is the answer
  • Detected time complexity: O(N) or O(N*log(N))