Moved StackPostfixNotation.java from the Others section to the Stack section (#4372)

* Moved StackPostfixNotation.java from the Others section to the Stack section

* Put all stack related algo in a separate stack directory in the algorithms directory. The stack directory under data-structures now only contains various implementations of the stack data structure.

* formatted files

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

@@ -0,0 +1,57 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class CalculateMaxOfMinTest {
+
+    @Test
+    void testForOneElement() {
+        int[] a = {10, 20, 30, 50, 10, 70, 30};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(70, k);
+    }
+
+    @Test
+    void testForTwoElements() {
+        int[] a = {5, 3, 2, 6, 3, 2, 6};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(6, k);
+    }
+
+    @Test
+    void testForThreeElements() {
+        int[] a = {10, 10, 10, 10, 10, 10, 10};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(10, k);
+    }
+
+    @Test
+    void testForFourElements() {
+        int[] a = {70, 60, 50, 40, 30, 20};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(70, k);
+    }
+
+    @Test
+    void testForFiveElements() {
+        int[] a = {50};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(50, k);
+    }
+
+    @Test
+    void testForSixElements() {
+        int[] a = {1, 4, 7, 9, 2, 4, 6};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(9, k);
+    }
+
+    @Test
+    void testForSevenElements() {
+        int[] a = {-1, -5, -7, -9, -12, -14};
+        int k = CalculateMaxOfMin.calculateMaxOfMin(a);
+        assertEquals(-1, k);
+    }
+}

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

@@ -0,0 +1,42 @@
+package com.thealgorithms.stacks;
+
+import static java.util.Map.entry;
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+public class StackPostfixNotationTest {
+    @Test
+    public void testEvaluate() {
+        final Map<String, Integer> testCases = Map.ofEntries(entry("1 1 +", 2), entry("2 3 *", 6), entry("6 2 /", 3), entry("-5 -2 -", -3), entry("5 2 + 3 *", 21), entry("-5", -5));
+        for (final var tc : testCases.entrySet()) {
+            assertEquals(tc.getValue(), StackPostfixNotation.postfixEvaluate(tc.getKey()));
+        }
+    }
+
+    @Test
+    public void testIfEvaluateThrowsExceptionForEmptyInput() {
+        assertThrows(IllegalArgumentException.class, () -> StackPostfixNotation.postfixEvaluate(""));
+    }
+
+    @Test
+    public void testIfEvaluateThrowsExceptionForInproperInput() {
+        assertThrows(IllegalArgumentException.class, () -> StackPostfixNotation.postfixEvaluate("3 3 3"));
+    }
+
+    @Test
+    public void testIfEvaluateThrowsExceptionForInputWithUnknownOperation() {
+        assertThrows(IllegalArgumentException.class, () -> StackPostfixNotation.postfixEvaluate("3 3 !"));
+    }
+
+    @Test
+    public void testIfEvaluateThrowsExceptionForInputWithTooFewArgsA() {
+        assertThrows(IllegalArgumentException.class, () -> StackPostfixNotation.postfixEvaluate("+"));
+    }
+
+    @Test
+    public void testIfEvaluateThrowsExceptionForInputWithTooFewArgsB() {
+        assertThrows(IllegalArgumentException.class, () -> StackPostfixNotation.postfixEvaluate("2 +"));
+    }
+}