I don't understand a test case for this problem https://codeforces.com/contest/1882/problem/B It is a relatively recent contest so it's still fresh in your minds. Look at the third test case. Isn't the union with most elements that is not equal to the s1 U s2 U s3... sn , is s2 U s3 U s5? Well this set contains 7 elements not 6, isn't it supposed to be {1,3,5,6,8,9,10} , what am I not understanding?