My algorithm solves a custom string problem but it loops twice making it $O(n^2)$ time. I'm asking here as I'm a beginner in self-learning algorithms.

Decision Problem: Given an input list $A$, can our target string $S$ be made from any of the elements of $A$?

If just one element does not exist in $A$ (which exists in $s$) then the algorithm returns false.

A = input_array
s = input_target_array

for i in s:
  if i in A:
    (all elements ∈ s) ∈ A

(if any element ∈ s) ∉ A:
  return False
  return True 


Enter your input for A and include spaces:
Enter letters WITH SPACES: "ab" "cd" "ac"
enter your input for s:
^ False ^
^ True ^

I'm looking for an algorithm that solves my made up decision problem faster than $O(n^2)$ time.


Are there any methods that can be used from an algorithmic standpoint that would make this $O(n)$ time?

  • 2
    $\begingroup$ How large is the alphabet? Should we treat the size of the alphabet as constant ($O(1)$) or not? $\endgroup$
    – D.W.
    Commented Jan 22, 2020 at 6:04
  • $\begingroup$ @D.W. The alphabet could be any of the infinite ones out there. Sometimes I use smiley faces and pineapples as a character string. $\endgroup$
    – The T
    Commented Jan 22, 2020 at 12:15
  • 1
    $\begingroup$ what does (all elements ∈ s) ∈ A mean in code? $\endgroup$ Commented Jan 22, 2020 at 12:55
  • 1
    $\begingroup$ stackoverflow.com/q/4642172/781723, cs.stackexchange.com/q/95996/755 $\endgroup$
    – D.W.
    Commented Jan 22, 2020 at 16:09
  • $\begingroup$ @user253751 Suppose that you see that the elements in $s$ as in the output example contains a character x. Since $A$ does not contain a character x then "(if any element ∈ s) ∉ A" is True (algorithm returns false). $\endgroup$
    – The T
    Commented Jan 22, 2020 at 22:14

1 Answer 1


I use set difference to find out if there are any elements in $s$ that don't exist in $A$. This should be $O(n)$ in the best cases if there is a good hash. Since I'm dealing with integers it should be $O(n)$ Explained here

I then use $len(s)$ and $len(A)$ which takes $O(1)$ time according to here.

A = [1,2,3,4,9]
s = [1,2,3,4,9]

elem_not_in_A = set(s) - set(A)

if len(s) >= len(A):
    if len(elem_not_in_A) > 0:
        output False
        output True
    output False

If the set difference remains $O(n)$ and all the other statements remain $O(1)$ then yes it can be solved in $O(n)$ time.


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