Python | Check if two lists have at-least one element common

Given two lists a, b. Check if two lists have at least one element common in them. 

Examples:

Input : a = [1, 2, 3, 4, 5]
        b = [5, 6, 7, 8, 9]
Output : True

Input : a=[1, 2, 3, 4, 5]
        b=[6, 7, 8, 9]
Output : False

Method 1: Traversal of List

Using traversal in two lists, we can check if there exists one common element at least in them. While traversing two lists if we find one element to be common in them, then we return true. After complete traversal and checking, if no elements are same, then we return false. 

Python

# Python program to check
# if two lists have at-least
# one element common
# using traversal of list

def common_data(list1, list2):
    result = False

    # traverse in the 1st list
    for x in list1:

        # traverse in the 2nd list
        for y in list2:

            # if one common
            if x == y:
                result = True
                return result

    return result

# driver code
a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(common_data(a, b))

a = [1, 2, 3, 4, 5]
b = [6, 7, 8, 9]
print(common_data(a, b))

Output:

True 
False

Time complexity: O(n*m), where n and m are the lengths of the input lists.
Auxiliary space: O(1).

Method 2 : Using Set and Property

Using set’s and property, if there exists at least one common element then set(a)&set(b) returns a positive integer, if it does not contains any positive integer, then it returns 0. So we insert a in set_a and b in set_b and then check if set_a & set_b for a positive integer or not. 

Python

# Python program to check
# if two lists have at-least
# one element common
# using set and property

def common_member(a, b):
    a_set = set(a)
    b_set = set(b)
    if (a_set & b_set):
        return True
    else:
        return False


a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(common_member(a, b))

a =[1, 2, 3, 4, 5]
b =[6, 7, 8, 9]
print(common_member(a, b))

Output:

True 
False

Time complexity: O(m + n), where m and n are the lengths of the input lists a and b, respectively.
Auxiliary space: O(m + n), for creating the two sets a_set and b_set.

Method 3 : Using Set Intersection

Using set’s intersection inbuilt function. a_set.intersection(b_set) returns a positive integer if there is at least one element in common, else it returns 0. So we insert a in set_a and b in set_b and then check a_set.intersection(b_set), and returns depending on the value. 

Python

# Python program to check
# if two lists have at-least
# one element common
# using set intersection

def common_member(a, b):
    a_set = set(a)
    b_set = set(b)
    if len(a_set.intersection(b_set)) > 0:
        return(True)
    return(False)

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(common_member(a, b))

a =[1, 2, 3, 4, 5]
b =[6, 7, 8, 9]
print(common_member(a, b))

Output:

True 
False

Time complexity: O(n), where n is the size of the larger list between a and b. 
Auxiliary space: O(n), where n is the size of the larger list between a and b.

Another approach that can be used to check if two lists have at least one common element is to use the Counter class from the collections module. The Counter class is a subclass of dict that is used to count the occurrences of elements in a list.

Here is an example of how the Counter class can be used to check if two lists have at least one common element:

Python3

from collections import Counter

def have_common_element(list1, list2):
    counter1 = Counter(list1)
    counter2 = Counter(list2)
    for element, count in counter1.items():
        if element in counter2 and count > 0:
            return True
    return False

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(have_common_element(a, b))  # True

a = [1, 2, 3, 4, 5]
b = [6, 7, 8, 9]
print(have_common_element(a, b))  # False

Output

True
False

Time complexity: O(n+m), where n is the length of list1 and m is the length of list2.

Auxiliary space: O(k), where k is the number of unique elements in both lists combined. 

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, 1, 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

True

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

Method 4: Using the any() function and the list comprehension method.

This methos uses the any() function along with list comprehension to check if two lists have at least one common element. The list comprehension creates a list of boolean values by comparing each element of the first list to each element of the second list. The any() function returns True if at least one of the boolean values in the list is True, indicating that there is at least one common element between the two lists.

Algorithm:

Create a boolean list using list comprehension that compares each element of the first list to each element of the second list.
Check if any() of the boolean values in the list is True.
If any() is True, return True. Otherwise, return False.

Python3

def common_member(a, b):
    return any(i in b for i in a)

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(common_member(a, b))  # True

a =[1, 2, 3, 4, 5]
b =[6, 7, 8, 9]
print(common_member(a, b))  # False

Output

True
False

Time complexity: O(n*m), where n and m are the lengths of the input lists.
Auxiliary space: O(1).