Word Frequency Counter

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One line for each word.
 * Approach: Count in a map, then sort.
 * Time    : O(n + k log k)
 * Space   : O(k)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        // Step 1: A HashMap counts the words. Key = word, value = how often it appeared.
        java.util.Map<String, Integer> counts = new java.util.HashMap<>();

        for (String line : lines) {
            // Step 2: Make everything lower case and cut the text at every character that is not a letter.
            for (String word : line.toLowerCase().split("[^a-z]+")) {
                if (!word.isEmpty()) {
                    counts.put(word, counts.getOrDefault(word, 0) + 1);
                }
            }
        }

        // Step 3: Sort the words. A higher count goes first. For equal counts the words are in alphabetical order.
        java.util.List<String> words = new java.util.ArrayList<>(counts.keySet());
        words.sort((first, second) -> {
            int byCount = counts.get(second) - counts.get(first);
            return byCount != 0 ? byCount : first.compareTo(second);
        });

        // Step 4: Make the output lines.
        String[] output = new String[words.size()];
        for (int i = 0; i < output.length; i++) {
            output[i] = words.get(i) + " " + counts.get(words.get(i));
        }
        return output;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}