feat: Add Hungarian Algorithm (Assignment Problem) with tests and reference [GRAPHS] (#6808)

* feat(graph): add Hungarian Algorithm (assignment) with tests; update directory

* style(graph): split multiple variable declarations to satisfy Checkstyle

src/test/java/com/thealgorithms/graph/HungarianAlgorithmTest.java

@@ -0,0 +1,44 @@
+package com.thealgorithms.graph;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+class HungarianAlgorithmTest {
+
+    @Test
+    @DisplayName("Classic 3x3 example: minimal cost 5 with assignment [1,0,2]")
+    void classicSquareExample() {
+        int[][] cost = {{4, 1, 3}, {2, 0, 5}, {3, 2, 2}};
+        HungarianAlgorithm.Result res = HungarianAlgorithm.solve(cost);
+        assertEquals(5, res.minCost);
+        assertArrayEquals(new int[] {1, 0, 2}, res.assignment);
+    }
+
+    @Test
+    @DisplayName("Rectangular (more rows than cols): pads to square and returns -1 for unassigned rows")
+    void rectangularMoreRows() {
+        int[][] cost = {{7, 3}, {2, 8}, {5, 1}};
+        // Optimal selects any 2 rows: choose row1->col0 (2) and row2->col1 (1) => total 3
+        HungarianAlgorithm.Result res = HungarianAlgorithm.solve(cost);
+        assertEquals(3, res.minCost);
+        // Two rows assigned to 2 columns; one row remains -1.
+        int assigned = 0;
+        for (int a : res.assignment) {
+            if (a >= 0) {
+                assigned++;
+            }
+        }
+        assertEquals(2, assigned);
+    }
+
+    @Test
+    @DisplayName("Zero diagonal yields zero total cost")
+    void zeroDiagonal() {
+        int[][] cost = {{0, 5, 9}, {4, 0, 7}, {3, 6, 0}};
+        HungarianAlgorithm.Result res = HungarianAlgorithm.solve(cost);
+        assertEquals(0, res.minCost);
+    }
+}