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/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 +"));
+    }
+}