this program finds the lowest unused number from an array

zero.py

someArray = [0, 1, 3]
contextSmallest = someArray[0] # considering the first element of the aray to be smallest

for i in range(1, len(someArray)): # finding the actual smallest number
    if someArray[i] < contextSmallest:
        contextSmallest = someArray[i]

smallestUnused = None # let's say

while smallestUnused is None:
    temp = contextSmallest + 1
    if not (temp in someArray): # if just one more than the smallest is not present in the array, it must be the smallest unused number
        smallestUnused = temp
    else:
        contextSmallest = temp

print("smallest unused is - ", smallestUnused)