Using parity bits in a single integer was a fun way to do this in C++. Here a 0 bit is even, a 1 bit is odd.

int main() {
    string s;
    cin >> s;
    int parity = 0;
    for (char ch : s)
        parity ^= 1 << (ch - 'a');
    cout << (__builtin_popcount(parity) <= 1 ? "YES" : "NO") << endl;
    return 0;
}