feat-Add MaximumProductSubarray

src/main/java/com/thealgorithms/dynamicprogramming/MaximumProductSubarray.java

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+package com.thealgorithms.dynamicprogramming;
+
+/**
+ * The MaximumProductSubarray class implements the algorithm to find the
+ * maximum product of a contiguous subarray within a given array of integers.
+ *
+ * <p>Given an array of integers (which may contain positive numbers, negative
+ * numbers, and zeros), this algorithm finds the contiguous subarray that has
+ * the largest product. The algorithm handles negative numbers efficiently by
+ * tracking both maximum and minimum products, since a negative number can turn
+ * a minimum product into a maximum product.</p>
+ *
+ * <p>This implementation uses a dynamic programming approach that runs in O(n)
+ * time complexity and O(1) space complexity, making it highly efficient for
+ * large arrays.</p>
+ */
+public final class MaximumProductSubarray {
+
+    private MaximumProductSubarray() {
+        // Prevent instantiation
+    }
+
+    /**
+     * Finds the maximum product of any contiguous subarray in the given array.
+     *
+     * @param nums an array of integers which may contain positive, negative,
+     *             and zero values.
+     * @return the maximum product of a contiguous subarray. Returns 0 if the
+     *         array is null or empty.
+     */
+    public static int maxProduct(int[] nums) {
+        if (nums == null || nums.length == 0) {
+            return 0;
+        }
+
+        long maxProduct = nums[0];
+        long currentMax = nums[0];
+        long currentMin = nums[0];
+
+        for (int i = 1; i < nums.length; i++) {
+            // Swap currentMax and currentMin if current number is negative
+            if (nums[i] < 0) {
+                long temp = currentMax;
+                currentMax = currentMin;
+                currentMin = temp;
+            }
+
+            // Update currentMax and currentMin
+            currentMax = Math.max(nums[i], currentMax * nums[i]);
+            currentMin = Math.min(nums[i], currentMin * nums[i]);
+
+            // Update global max product
+            maxProduct = Math.max(maxProduct, currentMax);
+        }
+
+        return (int) maxProduct;
+    }
+}

src/test/java/com/thealgorithms/dynamicprogramming/MaximumProductSubarrayTest.java

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+package com.thealgorithms.dynamicprogramming;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class MaximumProductSubarrayTest {
+
+    /**
+     * Test case for an array with all positive numbers.
+     * The expected maximum product is the product of all elements.
+     */
+    @Test
+    void testAllPositiveNumbers() {
+        int[] nums = {2, 3, 4};
+        int expected = 24;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 24.");
+    }
+
+    /**
+     * Test case for an array with positive and negative numbers.
+     * The expected maximum product is 24 (subarray [2, -3, -4]).
+     */
+    @Test
+    void testMixedPositiveAndNegative() {
+        int[] nums = {2, -3, -4, 1};
+        int expected = 24;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 24.");
+    }
+
+    /**
+     * Test case for an array containing zeros.
+     * The expected maximum product is 24 (subarray [4, 6]).
+     */
+    @Test
+    void testArrayWithZeros() {
+        int[] nums = {2, 3, 0, 4, 6};
+        int expected = 24;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 24.");
+    }
+
+    /**
+     * Test case for an array with a single element.
+     * The expected maximum product is the element itself.
+     */
+    @Test
+    void testSingleElement() {
+        int[] nums = {5};
+        int expected = 5;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 5.");
+    }
+
+    /**
+     * Test case for an array with all negative numbers.
+     * The expected maximum product is 12 (subarray [-3, -4]).
+     */
+    @Test
+    void testAllNegativeNumbers() {
+        int[] nums = {-2, -3, -4};
+        int expected = 12;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 12.");
+    }
+
+    /**
+     * Test case for an array with negative numbers where odd count of negatives
+     * breaks the chain. The expected maximum product is 60 (subarray [-2, -3, 10]).
+     */
+    @Test
+    void testOddNegativeNumbers() {
+        int[] nums = {-2, -3, 10, -1};
+        int expected = 60;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 60.");
+    }
+
+    /**
+     * Test case for an empty array.
+     * The expected maximum product is 0.
+     */
+    @Test
+    void testEmptyArray() {
+        int[] nums = {};
+        int expected = 0;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 0 for an empty array.");
+    }
+
+    /**
+     * Test case for a null array.
+     * The expected maximum product is 0.
+     */
+    @Test
+    void testNullArray() {
+        int[] nums = null;
+        int expected = 0;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 0 for a null array.");
+    }
+
+    /**
+     * Test case for an array with alternating positive and negative numbers.
+     * The expected maximum product is 6 (subarray [2, 3]).
+     */
+    @Test
+    void testAlternatingNumbers() {
+        int[] nums = {2, 3, -2, 4};
+        int expected = 6;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 6.");
+    }
+
+    /**
+     * Test case for an array with large positive and negative numbers.
+     * The expected maximum product is 360 (subarray [6, -3, -20]).
+     */
+    @Test
+    void testLargeNumbers() {
+        int[] nums = {6, -3, -20, 0, 5};
+        int expected = 360;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 360.");
+    }
+
+    /**
+     * Test case for an array with single negative number.
+     * The expected maximum product is the negative number itself.
+     */
+    @Test
+    void testSingleNegativeElement() {
+        int[] nums = {-8};
+        int expected = -8;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be -8.");
+    }
+
+    /**
+     * Test case for an array with multiple zeros.
+     * The expected maximum product is 6 (subarray [2, 3]).
+     */
+    @Test
+    void testMultipleZeros() {
+        int[] nums = {0, 2, 3, 0, 4};
+        int expected = 6;
+        int actual = MaximumProductSubarray.maxProduct(nums);
+        assertEquals(expected, actual, "The maximum product should be 6.");
+    }
+}