Add Sliding Window algorithm and tests for maximum sum of subarray

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

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+package com.thealgorithms.slidingwindow;
+
+/**
+ * The Sliding Window algorithm is used to find the maximum sum of a subarray
+ * of a fixed size k within a given array.
+ *
+ * <p>
+ * Worst-case performance O(n)
+ * Best-case performance O(n)
+ * Average performance O(n)
+ * Worst-case space complexity O(1)
+ *
+ * @author Your Name (https://github.com/Chiefpatwal)
+ */
+public final class MaxSumKSizeSubarray {
+
+    // Prevent instantiation
+    private MaxSumKSizeSubarray() {
+    }
+
+    /**
+     * This method finds the maximum sum of a subarray of a given size k.
+     *
+     * @param arr is the input array where the maximum sum needs to be found
+     * @param k   is the size of the subarray
+     * @return the maximum sum of the subarray of size k
+     */
+    public static int maxSumKSizeSubarray(int[] arr, int k) {
+        if (arr.length < k) {
+            return -1; // Edge case: not enough elements
+        }
+
+        int maxSum;
+        int windowSum = 0;
+
+        // Calculate the sum of the first window
+        for (int i = 0; i < k; i++) {
+            windowSum += arr[i];
+        }
+        maxSum = windowSum;
+
+        // Slide the window across the array
+        for (int i = k; i < arr.length; i++) {
+            windowSum += arr[i] - arr[i - k];
+            maxSum = Math.max(maxSum, windowSum);
+        }
+
+        return maxSum;
+    }
+}

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

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+package com.thealgorithms.slidingwindow;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the MaxSumKSizeSubarray class.
+ *
+ * @author Your Name (https://github.com/Chiefpatwal)
+ */
+class MaxSumKSizeSubarrayTest {
+
+    /**
+     * Test for the basic case of finding the maximum sum.
+     */
+    @Test
+    void testMaxSumKSizeSubarray() {
+        int[] arr = {1, 2, 3, 4, 5};
+        int k = 2;
+        int expectedMaxSum = 9; // 4 + 5
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+
+    /**
+     * Test for a different array and subarray size.
+     */
+    @Test
+    void testMaxSumKSizeSubarrayWithDifferentValues() {
+        int[] arr = {2, 1, 5, 1, 3, 2};
+        int k = 3;
+        int expectedMaxSum = 9; // 5 + 1 + 3
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+
+    /**
+     * Test for edge case with insufficient elements.
+     */
+    @Test
+    void testMaxSumKSizeSubarrayWithInsufficientElements() {
+        int[] arr = {1, 2};
+        int k = 3; // Not enough elements
+        int expectedMaxSum = -1; // Edge case
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+
+    /**
+     * Test for large array.
+     */
+    @Test
+    void testMaxSumKSizeSubarrayWithLargeArray() {
+        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        int k = 5;
+        int expectedMaxSum = 40; // 6 + 7 + 8 + 9 + 10
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+
+    /**
+     * Test for array with negative numbers.
+     */
+    @Test
+    void testMaxSumKSizeSubarrayWithNegativeNumbers() {
+        int[] arr = {-1, -2, -3, -4, -5};
+        int k = 2;
+        int expectedMaxSum = -3; // -1 + -2
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+
+    /**
+     * Test for the case where k equals the array length.
+     */
+    @Test
+    void testMaxSumKSizeSubarrayWithKEqualToArrayLength() {
+        int[] arr = {1, 2, 3, 4, 5};
+        int k = 5;
+        int expectedMaxSum = 15; // 1 + 2 + 3 + 4 + 5
+        assertEquals(expectedMaxSum, MaxSumKSizeSubarray.maxSumKSizeSubarray(arr, k));
+    }
+}