Handle inputs like "2 +" in StackPostfixNotation (#4262)

Author: vil02

src/main/java/com/thealgorithms/others/StackPostfixNotation.java

@@ -16,6 +16,9 @@ public static int postfixEvaluate(final String exp) {
             if (tokens.hasNextInt()) {
                 s.push(tokens.nextInt()); // If int then push to stack
             } else { // else pop top two values and perform the operation
+                if (s.size() < 2) {
+                    throw new IllegalArgumentException("exp is not a proper postfix expression (too few arguments).");
+                }
                 int num2 = s.pop();
                 int num1 = s.pop();
                 String op = tokens.next();

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

@@ -30,4 +30,14 @@ public void testIfEvaluateThrowsExceptionForInproperInput() {
     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 +"));
+    }
 }