Add Sliding Window algorithm and tests for maximum sum of subarray (#6001)

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

@@ -0,0 +1,50 @@
+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;
+    }
+}