How to change a List to a Dictionary having the values as Boolean

I have the following code. In this case I have a Big O(n^2). How can I write this code in a better way using another Big O Notation? The code is suppose to return True if one value from the list matches one value from another, else it should return False.

l1 = [1, 2, 3, 4]
l2 = [1, 6, 7, 8]

def common_inputs(list1, list2):
  for i in l1:
    for j in l2:
      if i == j:
        return True
        return False

print(common_inputs(l1, l2))

I have an example but is written in JavaScript and can’t understand that much.

array1 = [1, 2, 3, 4]
array2 = [1, 6, 7, 8]

function commonInputs(arr1, arr2) {
  let map = {};
  for (let i=0; i < arr1.length; i++) {
    if (!map[i]) {
      const item = arr1[i];
      map[item] = true;

  for (let j=0; j < arr2.length; j++) {
    if (map[arr2[j]]) {
        return true;
  return false;

Any advice will be appreciated. Thanks in advance.


The python idiom to test if two sets intersect would be

intersect = not set(a1).isdisjoint(a2)

In javascript there’s no such thing, so you’ll have to loop the second list:

function intersect(a1, a2) {
    let s1 = new Set(a1)
    return a2.some(x => s1.has(x))