feat(slidingwindow): add SmallestSubarrayWithSum and its tests

Author: nihalsingh373

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

@@ -0,0 +1,47 @@
+package com.thealgorithms.slidingwindow;
+
+/**
+ * Finds the length of the smallest contiguous subarray
+ * whose sum is greater than or equal to a given target.
+ *
+ * <p>Example:
+ * arr = [2, 3, 1, 2, 4, 3], target = 7
+ * Smallest subarray = [4, 3], length = 2
+ *
+ * <p>Time complexity: O(n)
+ * Space complexity: O(1)
+ */
+public final class SmallestSubarrayWithSum {
+
+    // Prevent instantiation
+    private SmallestSubarrayWithSum() {}
+
+    /**
+     * Returns the minimal length of a contiguous subarray of which
+     * the sum ≥ target. If no such subarray exists, returns 0.
+     *
+     * @param arr the input array
+     * @param target the target sum
+     * @return the minimal subarray length, or 0 if not found
+     */
+    public static int smallestSubarrayLen(int[] arr, int target) {
+        if (arr == null || arr.length == 0) {
+            return 0;
+        }
+
+        int left = 0;
+        int sum = 0;
+        int minLen = Integer.MAX_VALUE;
+
+        for (int right = 0; right < arr.length; right++) {
+            sum += arr[right];
+            while (sum >= target) {
+                minLen = Math.min(minLen, right - left + 1);
+                sum -= arr[left++];
+            }
+        }
+
+        return minLen == Integer.MAX_VALUE ? 0 : minLen;
+    }
+}
+

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

@@ -0,0 +1,50 @@
+package com.thealgorithms.slidingwindow;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link SmallestSubarrayWithSum}.
+ */
+public class SmallestSubarrayWithSumTest {
+
+    @Test
+    public void testBasicCase() {
+        int[] arr = {2, 3, 1, 2, 4, 3};
+        int target = 7;
+        int expected = 2; // subarray [4, 3]
+        assertEquals(expected, SmallestSubarrayWithSum.smallestSubarrayLen(arr, target));
+    }
+
+    @Test
+    public void testNoValidSubarray() {
+        int[] arr = {1, 1, 1, 1};
+        int target = 10;
+        int expected = 0;
+        assertEquals(expected, SmallestSubarrayWithSum.smallestSubarrayLen(arr, target));
+    }
+
+    @Test
+    public void testSingleElement() {
+        int[] arr = {5};
+        int target = 5;
+        int expected = 1;
+        assertEquals(expected, SmallestSubarrayWithSum.smallestSubarrayLen(arr, target));
+    }
+
+    @Test
+    public void testAllElementsSame() {
+        int[] arr = {2, 2, 2, 2, 2};
+        int target = 6;
+        int expected = 3;
+        assertEquals(expected, SmallestSubarrayWithSum.smallestSubarrayLen(arr, target));
+    }
+
+    @Test
+    public void testLargeInput() {
+        int[] arr = {1, 2, 3, 4, 5, 6};
+        int target = 11;
+        int expected = 2; // subarray [5, 6]
+        assertEquals(expected, SmallestSubarrayWithSum.smallestSubarrayLen(arr, target));
+    }
+}