feat: add StockSpanProblem algorithm

src/main/java/com/thealgorithms/stacks/StockSpanProblem.java

@@ -0,0 +1,42 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ * Calculates the stock span for each day in a series of stock prices.
+ *
+ * <p>The span of a price on a given day is the number of consecutive days ending on that day
+ * for which the price was less than or equal to the current day's price.
+ *
+ */
+public final class StockSpanProblem {
+    private StockSpanProblem() {
+    }
+
+    /**
+     * Calculates the stock span for each price in the input array.
+     *
+     * @param prices the stock prices
+     * @return the span for each day
+     * @throws IllegalArgumentException if the input array is null
+     */
+    public static int[] calculateSpan(int[] prices) {
+        if (prices == null) {
+            throw new IllegalArgumentException("Input prices cannot be null");
+        }
+
+        int[] spans = new int[prices.length];
+        Stack<Integer> stack = new Stack<>();
+
+        for (int index = 0; index < prices.length; index++) {
+            while (!stack.isEmpty() && prices[stack.peek()] <= prices[index]) {
+                stack.pop();
+            }
+
+            spans[index] = stack.isEmpty() ? index + 1 : index - stack.peek();
+            stack.push(index);
+        }
+
+        return spans;
+    }
+}

src/test/java/com/thealgorithms/stacks/StockSpanProblemTest.java

@@ -0,0 +1,34 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class StockSpanProblemTest {
+
+    @ParameterizedTest
+    @MethodSource("validTestCases")
+    void testCalculateSpan(int[] prices, int[] expectedSpans) {
+        assertArrayEquals(expectedSpans, StockSpanProblem.calculateSpan(prices));
+    }
+
+    private static Stream<Arguments> validTestCases() {
+        return Stream.of(Arguments.of(new int[] {10, 4, 5, 90, 120, 80}, new int[] {1, 1, 2, 4, 5, 1}), Arguments.of(new int[] {100, 50, 60, 70, 80, 90}, new int[] {1, 1, 2, 3, 4, 5}), Arguments.of(new int[] {5, 4, 3, 2, 1}, new int[] {1, 1, 1, 1, 1}),
+            Arguments.of(new int[] {1, 2, 3, 4, 5}, new int[] {1, 2, 3, 4, 5}), Arguments.of(new int[] {10, 20, 30, 40, 50}, new int[] {1, 2, 3, 4, 5}), Arguments.of(new int[] {100, 80, 60, 70, 60, 75, 85}, new int[] {1, 1, 1, 2, 1, 4, 6}),
+            Arguments.of(new int[] {7, 7, 7, 7}, new int[] {1, 2, 3, 4}), Arguments.of(new int[] {}, new int[] {}), Arguments.of(new int[] {42}, new int[] {1}));
+    }
+
+    @ParameterizedTest
+    @MethodSource("invalidTestCases")
+    void testCalculateSpanInvalidInput(int[] prices) {
+        assertThrows(IllegalArgumentException.class, () -> StockSpanProblem.calculateSpan(prices));
+    }
+
+    private static Stream<Arguments> invalidTestCases() {
+        return Stream.of(Arguments.of((int[]) null));
+    }
+}