Solution to "Count Vowel Strings in Ranges" problem

src/main/java/com/github/dkoval/leetcode/challenge/CountVowelStringsInRanges.java

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+package com.github.dkoval.leetcode.challenge;
+
+import java.util.Set;
+
+/**
+ * <a href="https://leetcode.com/problems/count-vowel-strings-in-ranges/">Count Vowel Strings in Ranges</a>
+ * <p>
+ * You are given a 0-indexed array of strings words and a 2D array of integers queries.
+ * <p>
+ * Each query queries[i] = [li, ri] asks us to find the number of strings present in the range li to ri (both inclusive)
+ * of words that start and end with a vowel.
+ * <p>
+ * Return an array ans of size queries.length, where ans[i] is the answer to the ith query.
+ * <p>
+ * Note that the vowel letters are 'a', 'e', 'i', 'o', and 'u'.
+ * <p>
+ * Constraints:
+ * <ul>
+ *  <li>1 <= words.length <= 10^5</li>
+ *  <li>1 <= words[i].length <= 4^0</li>
+ *  <li>words[i] consists only of lowercase English letters.</li>
+ *  <li>sum(words[i].length) <= 3 * 10^5</li>
+ *  <li>1 <= queries.length <= 10^5</li>
+ *  <li>0 <= li <= ri < words.length</li>
+ * </ul>
+ */
+public interface CountVowelStringsInRanges {
+
+    int[] vowelStrings(String[] words, int[][] queries);
+
+    class CountVowelStringsInRangesRev1 implements CountVowelStringsInRanges {
+
+        private static final Set<Character> VOWELS = Set.of('a', 'e', 'i', 'o', 'u');
+
+        @Override
+        public int[] vowelStrings(String[] words, int[][] queries) {
+            final var n = words.length;
+            final var q = queries.length;
+
+            // prefix[i] - the number of "good" strings that end at index i
+            final var prefix = new int[n];
+            for (var i = 0; i < n; i++) {
+                if (i > 0) {
+                    prefix[i] = prefix[i - 1];
+                }
+                if (good(words[i])) {
+                    prefix[i]++;
+                }
+            }
+
+            final var ans = new int[q];
+            for (var i = 0; i < q; i++) {
+                final var l = queries[i][0];
+                final var r = queries[i][1];
+                ans[i] = prefix[r] - (l > 0 ? prefix[l - 1] : 0);
+            }
+            return ans;
+        }
+
+        private boolean good(String word) {
+            final var w = word.length();
+            return VOWELS.contains(word.charAt(0)) && VOWELS.contains(word.charAt(w - 1));
+        }
+    }
+}

src/test/kotlin/com/github/dkoval/leetcode/challenge/CountVowelStringsInRangesTest.kt

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+package com.github.dkoval.leetcode.challenge
+
+import com.github.dkoval.leetcode.challenge.CountVowelStringsInRanges.CountVowelStringsInRangesRev1
+import org.junit.jupiter.api.Assertions.assertArrayEquals
+import org.junit.jupiter.api.Nested
+import org.junit.jupiter.api.extension.ExtensionContext
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.ArgumentsProvider
+import org.junit.jupiter.params.provider.ArgumentsSource
+import java.util.stream.Stream
+
+internal class CountVowelStringsInRangesTest {
+
+    class InputArgumentsProvider : ArgumentsProvider {
+        override fun provideArguments(context: ExtensionContext): Stream<out Arguments> = Stream.of(
+            Arguments.of(
+                arrayOf("aba", "bcb", "ece", "aa", "e"),
+                arrayOf(
+                    intArrayOf(0, 2),
+                    intArrayOf(1, 4),
+                    intArrayOf(1, 1)
+                ),
+                intArrayOf(2, 3, 0)
+            ),
+            Arguments.of(
+                arrayOf("a", "e", "i"),
+                arrayOf(
+                    intArrayOf(0, 2),
+                    intArrayOf(0, 1),
+                    intArrayOf(2, 2)
+                ),
+                intArrayOf(3, 2, 1)
+            )
+        )
+    }
+
+    @Nested
+    inner class CountVowelStringsInRangesRev1Test {
+
+        @ParameterizedTest
+        @ArgumentsSource(InputArgumentsProvider::class)
+        fun `should return answers to the queries`(words: Array<String>, queries: Array<IntArray>, expected: IntArray) {
+            CountVowelStringsInRangesRev1().test(words, queries, expected)
+        }
+    }
+}
+
+private fun CountVowelStringsInRanges.test(words: Array<String>, queries: Array<IntArray>, expected: IntArray) {
+    val actual = vowelStrings(words, queries)
+    assertArrayEquals(expected, actual)
+}