Most Common Word — Normalize + Frequency Map

# Python
import re
from collections import Counter
def mostCommonWord(paragraph, banned):
    banned = set(banned)
    words = re.findall(r'[a-z]+', paragraph.lower())
    return Counter(w for w in words if w not in banned).most_common(1)[0][0]

// Java
public String mostCommonWord(String paragraph, String[] banned) {
    Set<String> ban = new HashSet<>(Arrays.asList(banned));
    String[] words = paragraph.toLowerCase().split("[^a-z]+");
    Map<String,Integer> freq = new HashMap<>();
    for (String w : words)
        if (!ban.contains(w)) freq.merge(w, 1, Integer::sum);
    return Collections.max(freq.entrySet(), Map.Entry.comparingByValue()).getKey();
}

// C++
string mostCommonWord(string paragraph, vector<string>& banned) {
    unordered_set<string> ban(banned.begin(), banned.end());
    for (char& c : paragraph) c = isalpha(c) ? tolower(c) : ' ';
    istringstream ss(paragraph);
    unordered_map<string,int> freq;
    string w, best; int mx = 0;
    while (ss >> w) {
        if (!ban.count(w) && ++freq[w] > mx) { mx = freq[w]; best = w; }
    }
    return best;
}