int sockMerchant(int n, vector<int> ar) {
    std::vector<bool> odds = std::vector<bool>(100);
    int nPairs = 0;
    for ( int sockID : ar ) {
        if ( odds[sockID-1] ) {
            nPairs++;
            odds[sockID-1] = false;
        } else {
            // odd sock found
            odds[sockID-1] = true;
        }
    }
    return nPairs;
}