Add suggested changes

Author: Hardvan

src/main/java/com/thealgorithms/stacks/PrefixToInfix.java

@@ -3,15 +3,12 @@
 import java.util.Stack;
 
 /**
- * Prefix to Infix implementation via Stack
+ * Converts a prefix expression to an infix expression using a stack.
  *
- * Function: String getPrefixToInfix(String prefix)
- * Returns the Infix Expression for the given prefix parameter.
- *
- * Avoid using parentheses/brackets/braces for the prefix string.
- * Prefix Expressions don't require these.
+ * The input prefix expression should consist of
+ * valid operands (letters or digits) and operators (+, -, *, /, ^).
+ * Parentheses are not required in the prefix string.
  */
-
 public final class PrefixToInfix {
     private PrefixToInfix() {
     }
@@ -63,6 +60,10 @@ public static String getPrefixToInfix(String prefix) {
             }
         }
 
+        if (stack.size() != 1) {
+            throw new ArithmeticException("Malformed prefix expression");
+        }
+
         return stack.pop(); // final element on the stack is the full infix expression
     }
 }

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

@@ -36,4 +36,9 @@ void testEmptyPrefixExpression() {
     void testNullPrefixExpression() {
         assertThrows(NullPointerException.class, () -> PrefixToInfix.getPrefixToInfix(null));
     }
+
+    @Test
+    void testMalformedPrefixExpression() {
+        assertThrows(ArithmeticException.class, () -> PrefixToInfix.getPrefixToInfix("+ABC"));
+    }
 }