Untitled

#include <iostream>
#include <vector>
#include <unordered_map>
using namespace std;

int countComplementaryPairs(vector<string>& words) {
    unordered_map<int, int> bitmask_count;
    int count_pairs = 0;

    for (const string& word : words) {
        int mask = 0;
        
        // Compute the bitmask for the current word
        for (char ch : word) {
            mask ^= (1 << (ch - 'a'));  // Toggle the corresponding bit
        }

        // Case 1: Count exact matches (same bitmask)
        count_pairs += bitmask_count[mask];

        // Case 2: Count bitmasks differing by exactly one bit
        for (int i = 0; i < 26; ++i) {
            count_pairs += bitmask_count[mask ^ (1 << i)];
        }

        // Store the current bitmask in the hashmap
        bitmask_count[mask]++;
    }

    return count_pairs;
}

// Driver function
int main() {
    vector<string> words = {"abac", "cab", "abc", "cba", "aa", "bb"};
    cout << "Number of complementary pairs: " << countComplementaryPairs(words) << endl;
    return 0;
}