Count Pairs of Strings with No Common Characters

Given an array words of n strings, count the number of index pairs (i, j) such that: 0 <= i < j < n words[i] and words[j] share no character. Every string contains only lowercase English letters. Example: Input: words = ["abcw", "baz", "foo", "bar", "xtfn", "abcdef"] Output: 3 Explanation: The valid pairs are (0, 2), (1, 4), and (3, 4). Constraints: 1 <= n <= 10^5 1 <= len(words[i]) <= 10^5 The sum of all string lengths is at most 10^6. Example Input 6 abcw baz foo bar xtfn abcdef Output 3