feat: add TopKFrequentWords with deterministic tie-breaking (#7298)

src/main/java/com/thealgorithms/strings/TopKFrequentWords.java

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+package com.thealgorithms.strings;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Utility class to find the top-k most frequent words.
+ *
+ * <p>Words are ranked by frequency in descending order. For equal frequencies,
+ * words are ranked in lexicographical ascending order.
+ *
+ * <p>Reference:
+ * https://en.wikipedia.org/wiki/Top-k_problem
+ *
+ */
+public final class TopKFrequentWords {
+    private TopKFrequentWords() {
+    }
+
+    /**
+     * Finds the k most frequent words.
+     *
+     * @param words input array of words
+     * @param k number of words to return
+     * @return list of top-k words ordered by frequency then lexicographical order
+     * @throws IllegalArgumentException if words is null, k is negative, or words contains null
+     */
+    public static List<String> findTopKFrequentWords(String[] words, int k) {
+        if (words == null) {
+            throw new IllegalArgumentException("Input words array cannot be null.");
+        }
+        if (k < 0) {
+            throw new IllegalArgumentException("k cannot be negative.");
+        }
+        if (k == 0 || words.length == 0) {
+            return List.of();
+        }
+
+        Map<String, Integer> frequency = new HashMap<>();
+        for (String word : words) {
+            if (word == null) {
+                throw new IllegalArgumentException("Input words cannot contain null values.");
+            }
+            frequency.put(word, frequency.getOrDefault(word, 0) + 1);
+        }
+
+        List<String> candidates = new ArrayList<>(frequency.keySet());
+        candidates.sort(Comparator.<String>comparingInt(frequency::get).reversed().thenComparing(Comparator.naturalOrder()));
+
+        int limit = Math.min(k, candidates.size());
+        return new ArrayList<>(candidates.subList(0, limit));
+    }
+}

src/test/java/com/thealgorithms/strings/TopKFrequentWordsTest.java

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+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.List;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class TopKFrequentWordsTest {
+
+    @ParameterizedTest
+    @MethodSource("validTestCases")
+    void testFindTopKFrequentWords(String[] words, int k, List<String> expected) {
+        assertEquals(expected, TopKFrequentWords.findTopKFrequentWords(words, k));
+    }
+
+    static Stream<Arguments> validTestCases() {
+        return Stream.of(Arguments.of(new String[] {"i", "love", "leetcode", "i", "love", "coding"}, 2, List.of("i", "love")), Arguments.of(new String[] {"the", "day", "is", "sunny", "the", "the", "the", "sunny", "is", "is"}, 4, List.of("the", "is", "sunny", "day")),
+            Arguments.of(new String[] {"bbb", "aaa", "bbb", "aaa", "ccc"}, 2, List.of("aaa", "bbb")), Arguments.of(new String[] {"one", "two", "three"}, 10, List.of("one", "three", "two")), Arguments.of(new String[] {}, 3, List.of()), Arguments.of(new String[] {"x", "x", "y"}, 0, List.of()));
+    }
+
+    @ParameterizedTest
+    @MethodSource("invalidTestCases")
+    void testFindTopKFrequentWordsInvalidInput(String[] words, int k) {
+        assertThrows(IllegalArgumentException.class, () -> TopKFrequentWords.findTopKFrequentWords(words, k));
+    }
+
+    static Stream<Arguments> invalidTestCases() {
+        return Stream.of(Arguments.of((String[]) null, 1), Arguments.of(new String[] {"a", null, "b"}, 2), Arguments.of(new String[] {"a"}, -1));
+    }
+}