Add Count Nice Subarrays sliding using window algorithm

src/main/java/com/thealgorithms/slidingwindow/CountNiceSubarrays.java

@@ -0,0 +1,99 @@
+package com.thealgorithms.slidingwindow;
+
+/**
+ * Counts the number of "nice subarrays".
+ * A nice subarray is a contiguous subarray that contains exactly k odd numbers.
+ *
+ * This implementation uses the sliding window technique.
+ *
+ * Reference:
+ * https://leetcode.com/problems/count-number-of-nice-subarrays/
+ *
+ * Time Complexity: O(n)
+ * Space Complexity: O(n)
+ */
+public final class CountNiceSubarrays {
+
+    // Private constructor to prevent instantiation
+    private CountNiceSubarrays() {
+    }
+
+    /**
+     * Returns the count of subarrays containing exactly k odd numbers.
+     *
+     * @param nums input array of integers
+     * @param k    number of odd elements required in the subarray
+     * @return number of nice subarrays
+     */
+    public static int countNiceSubarrays(int[] nums, int k) {
+
+        int n = nums.length;
+
+        // Left pointer of the sliding window
+        int left = 0;
+
+        // Tracks number of odd elements in the current window
+        int oddCount = 0;
+
+        // Final answer: total number of nice subarrays
+        int result = 0;
+
+        /*
+         * memo[i] stores how many valid starting positions exist
+         * when the left pointer is at index i.
+         *
+         * This avoids recomputing the same values again.
+         */
+        int[] memo = new int[n];
+
+        // Right pointer moves forward to expand the window
+        for (int right = 0; right < n; right++) {
+
+            // If current element is odd, increment odd count
+            if ((nums[right] & 1) == 1) {
+                oddCount++;
+            }
+
+            /*
+             * If oddCount exceeds k, shrink the window from the left
+             * until oddCount becomes valid again.
+             */
+            if (oddCount > k) {
+                left += memo[left];
+                oddCount--;
+            }
+
+            /*
+             * When the window contains exactly k odd numbers,
+             * count all possible valid subarrays starting at `left`.
+             */
+            if (oddCount == k) {
+
+                /*
+                 * If this left index hasn't been processed before,
+                 * count how many consecutive even numbers follow it.
+                 */
+                if (memo[left] == 0) {
+                    int count = 0;
+                    int temp = left;
+
+                    // Count consecutive even numbers
+                    while ((nums[temp] & 1) == 0) {
+                        count++;
+                        temp++;
+                    }
+
+                    /*
+                     * Number of valid subarrays starting at `left`
+                     * is (count of even numbers + 1)
+                     */
+                    memo[left] = count + 1;
+                }
+
+                // Add number of valid subarrays for this left position
+                result += memo[left];
+            }
+        }
+        return result;
+    }
+}

src/test/java/com/thealgorithms/slidingwindow/CountNiceSubarraysTest.java

@@ -0,0 +1,55 @@
+package com.thealgorithms.slidingwindow;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class CountNiceSubarraysTest {
+    @Test
+    void testExampleCase() {
+        int[] nums = {1, 1, 2, 1, 1};
+        assertEquals(2, CountNiceSubarrays.countNiceSubarrays(nums, 3));
+    }
+
+    @Test
+    void testAllEvenNumbers() {
+        int[] nums = {2, 4, 6, 8};
+        assertEquals(0, CountNiceSubarrays.countNiceSubarrays(nums, 1));
+    }
+
+    @Test
+    void testSingleOdd() {
+        int[] nums = {1};
+        assertEquals(1, CountNiceSubarrays.countNiceSubarrays(nums, 1));
+    }
+
+    @Test
+    void testMultipleChoices() {
+        int[] nums = {2, 2, 1, 2, 2, 1, 2};
+        assertEquals(6, CountNiceSubarrays.countNiceSubarrays(nums, 2));
+    }
+
+    @Test
+    void testTrailingEvenNumbers() {
+        int[] nums = {1, 2, 2, 2};
+        assertEquals(4, CountNiceSubarrays.countNiceSubarrays(nums, 1));
+    }
+
+    @Test
+    void testMultipleWindowShrinks() {
+        int[] nums = {1, 1, 1, 1};
+        assertEquals(3, CountNiceSubarrays.countNiceSubarrays(nums, 2));
+    }
+
+    @Test
+    void testEvensBetweenOdds() {
+        int[] nums = {2, 1, 2, 1, 2};
+        assertEquals(4, CountNiceSubarrays.countNiceSubarrays(nums, 2));
+    }
+
+    @Test
+    void testShrinkWithTrailingEvens() {
+        int[] nums = {2, 2, 1, 2, 2, 1, 2, 2};
+        assertEquals(9, CountNiceSubarrays.countNiceSubarrays(nums, 2));
+    }
+}