refactor: InfixToPostfix (#5401)

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

@@ -0,0 +1,33 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class InfixToPostfixTest {
+
+    @ParameterizedTest
+    @MethodSource("provideValidExpressions")
+    void testValidExpressions(String infix, String expectedPostfix) throws Exception {
+        assertEquals(expectedPostfix, InfixToPostfix.infix2PostFix(infix));
+    }
+
+    private static Stream<Arguments> provideValidExpressions() {
+        return Stream.of(Arguments.of("3+2", "32+"), Arguments.of("1+(2+3)", "123++"), Arguments.of("(3+4)*5-6", "34+5*6-"));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideInvalidExpressions")
+    void testInvalidExpressions(String infix, String expectedMessage) {
+        Exception exception = assertThrows(Exception.class, () -> InfixToPostfix.infix2PostFix(infix));
+        assertEquals(expectedMessage, exception.getMessage());
+    }
+
+    private static Stream<Arguments> provideInvalidExpressions() {
+        return Stream.of(Arguments.of("((a+b)*c-d", "invalid expression"));
+    }
+}