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);
+    }
+}