refactor: PostfixToInfix (#5409)

* refactor: PostfixToInfix

* checkstyle: fix formatting

* refactor: add support for one character

---------

Co-authored-by: alxkm <alx@alx.com>

src/test/java/com/thealgorithms/stacks/PostfixToInfixTest.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.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class PostfixToInfixTest {
+
+    @ParameterizedTest
+    @MethodSource("provideValidPostfixToInfixTestCases")
+    void testValidPostfixToInfixConversion(String postfix, String expectedInfix) {
+        assertEquals(expectedInfix, PostfixToInfix.getPostfixToInfix(postfix));
+    }
+
+    static Stream<Arguments> provideValidPostfixToInfixTestCases() {
+        return Stream.of(Arguments.of("A", "A"), Arguments.of("ABC+/", "(A/(B+C))"), Arguments.of("AB+CD+*", "((A+B)*(C+D))"), Arguments.of("AB+C+D+", "(((A+B)+C)+D)"), Arguments.of("ABCDE^*/-", "(A-(B/(C*(D^E))))"), Arguments.of("AB+CD^/E*FGH+-^", "((((A+B)/(C^D))*E)^(F-(G+H)))"));
+    }
+
+    @Test
+    void testEmptyPostfixExpression() {
+        assertEquals("", PostfixToInfix.getPostfixToInfix(""));
+    }
+
+    @Test
+    void testNullPostfixExpression() {
+        assertThrows(NullPointerException.class, () -> PostfixToInfix.getPostfixToInfix(null));
+    }
+}