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int length_of_last_word(const string& s) {
    if(s.empty()) return 0;
    int N = s.size();
    int i = N-1;
    
    while(i >= 0 && s[i] == ' ') i--;
    if(i < 0) return 0;
    int end = i;
    while(i >= 0 && is_letter(s[i])) i--;
    return end - i;
}