Python | Check if two lists have any element in common

Sometimes we encounter the problem of checking if one list contains any element of another list. This kind of problem is quite popular in competitive programming. Let’s discuss various ways to achieve this particular task.

Method #1: Using any() 

Python3

# Python code to check if two lists
# have any element in common

# Initialization of list
list1 = [1, 2, 3, 4, 55]
list2 = [2, 3, 90, 22]

# using any function
out = any(check in list1 for check in list2)

# Checking condition
if out:
    print("True")
else :
    print("False")

Output:

True

Time complexity: O(n*m), where n and m are the lengths of list1 and list2 respectively. 
Auxiliary space: O(1), as only a few variables are used and no additional data structures are created.

Method #2: Using in operator. 

Python3

# Python code to check if two lists
# have any element in common

# Initialization of list
list1 = [1, 3, 4, 55]
list2 = [90, 22]

flag = 0

# Using in to check element of
# second list into first list
for elem in list2:
    if elem in list1:
        flag = 1

# checking condition
if flag == 1:
    print("True")
else :
    print("False")

Output:

False

Time Complexity: O(n), where n is length of list.

Auxiliary Space: O(1)

Method#3: Using set()

Python3

# Python code to check if two lists
# have any element in common

# Initialization of list
list1 = [1, 2, 3, 4, 55]
list2 = [2, 3, 90, 22]

# using set
out = set(list1) & set(list2)

# Checking condition
if out:
    print("True")
else :
    print("False")

Output

True

Time complexity: O(n*m), where n and m are the lengths of list1 and list2 respectively. 
Auxiliary space: O(1), as only a few variables are used and no additional data structures are created.

Method#4: Using intersection() method

Another approach to check if two lists have any elements in common is to use the built-in intersection function of the set class. This function returns a new set that contains the common elements of the two sets. If the intersection of the two sets is not empty, then it means that the lists have at least one element in common. Here’s an example of how this could be implemented:

Python3

# Initialization of list
list1 = [1, 2, 3, 4, 55]
list2 = [2, 3, 90, 22]
#Finding intersection
common_elements = set(list1).intersection(list2)

if common_elements:
    print("True")
else:
    print("False")
#This code is contributed by Edula Vinay Kumar Reddy

Output

True

Time complexity: O(n), where n is the length of the smaller list. This is because the intersection function iterates through each element of the smaller list and checks if it is present in the other list.
Auxiliary space: O(n), because the intersection function creates a new set that contains the common elements of the two lists, which has a size equal to the number of common elements.

Method #5 : Using operator.countOf() method

Python3

import operator as op
# Python code to check if two lists
# have any element in common

# Initialization of list
list1 = [1, 3, 4, 55]
list2 = [90, 22]

flag = 0

# Using in to check element of
# second list into first list
for elem in list2:
    if op.countOf(list1,elem)>0:
        flag = 1

# checking condition
if flag == 1:
    print("True")
else :
    print("False")

Output

False

Time Complexity: O(N)
Auxiliary Space: O(1)

Method #6: Using recursion

Step-by-step algorithm for the code:

1. Define a function find_common_elements that takes in two lists list1 and list2 as arguments.
2. Check if either of the lists is empty. If yes, return False.
3. Check if the first element of list1 is in list2. If yes, return True.
4. If the first element of list1 is not in list2, make a recursive call to the function by passing the remaining elements of list1 and list2 as arguments.
5. If no common element is found after checking all the elements of list1, return False.
6. Define two lists list1 and list2.
7. Call the find_common_elements function with the above-defined lists.
8. If the function returns True, print “True”. Else, print “False”.

End of program.

Python3

# Python code to check if two lists
# have any element in common

def find_common_elements(list1, list2):
    """
    Finds if two lists have any common element

    Args:
    list1, list2: input lists

    Returns:
    True if a common element exists, else False
    """
    # Using recursion
    if not list1 or not list2:
        return False
    if list1[0] in list2:
        return True
    return find_common_elements(list1[1:], list2)

# Example usage
list1 = [1, 2, 3, 4, 55]
list2 = [2, 3, 90, 22]
if find_common_elements(list1, list2):
    print("True")
else:
    print("False")
#This code is contributed by vinay Pinjala.

Output

True

Time complexity: O(m * n) in the worst case, where m and n are the lengths of the input lists. This is because the function checks each element of the first list against each element of the second list. In the best case, when the first element of the first list matches the first element of the second list, the function returns in O(1) time.

The space complexity  O(m) in the worst case, where m is the length of the longer list. This is because the function makes recursive calls with smaller lists, so at any point in time, there are at most m elements in the call stack. In the best case, when the two lists have no common elements, the function returns in O(1) space.