add two sum problem (#4364)

* add two sum problem

* linter solved

* linter solved

* improve code

* linter solved

* improve code

* mini linter solved

* update code

---------

Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>

src/test/java/com/thealgorithms/misc/TwoSumProblemTest.java

@@ -0,0 +1,61 @@
+package com.thealgorithms.misc;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
+import org.apache.commons.lang3.tuple.Pair;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test case for Two sum Problem.
+ * @author Bama Charan Chhandogi (https://github.com/BamaCharanChhandogi)
+ */
+
+public class TwoSumProblemTest {
+
+    @Test
+    void testTwoSumExists() {
+        final int[] values = new int[] {2, 7, 11, 15};
+        final int target = 9;
+        final var expected = Pair.of(0, 1); // values[0] + values[1] = 2 + 7 = 9
+        assertEquals(expected, TwoSumProblem.twoSum(values, target).get());
+    }
+
+    @Test
+    void testTwoSumNoSolution() {
+        final int[] values = new int[] {2, 7, 11, 15};
+        final int target = 3;
+        assertFalse(TwoSumProblem.twoSum(values, target).isPresent());
+    }
+
+    @Test
+    void testTwoSumMultipleSolutions() {
+        final int[] values = {3, 3};
+        final int target = 6;
+        final var expected = Pair.of(0, 1); // values[0] + values[1] = 3 + 3 = 6
+        assertEquals(expected, TwoSumProblem.twoSum(values, target).get());
+    }
+
+    @Test
+    void testTwoSumMultipleSolution() {
+        final int[] values = {3, 4, 3, 3};
+        final int target = 6;
+        final var expected = Pair.of(0, 2); // values[0] + values[2] = 3 + 3 = 6
+        assertEquals(expected, TwoSumProblem.twoSum(values, target).get());
+    }
+
+    @Test
+    void testTwoSumNegativeNumbers() {
+        final int[] values = {-1, -2, -3, -4, -5};
+        final int target = -8;
+        final var expected = Pair.of(2, 4); // values[2] + values[4] = -3 + (-5) = -8
+        assertEquals(expected, TwoSumProblem.twoSum(values, target).get());
+    }
+
+    @Test
+    void testTwoSumNoSolutionDuplicatedInputs() {
+        final int[] values = {0, 0, 0};
+        final int target = 100;
+        assertFalse(TwoSumProblem.twoSum(values, target).isPresent());
+    }
+}