Add PrefixToInfix.java new algorithm (#5552)

* Add `PrefixToInfix.java` new algorithm

* Update directory

* Fix clang error

* Update directory

* Fix comment

* Add suggested changes

---------

Co-authored-by: Hardvan <Hardvan@users.noreply.github.com>
Co-authored-by: Alex Klymenko <alexanderklmn@gmail.com>

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

@@ -0,0 +1,69 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ * Converts a prefix expression to an infix expression using a stack.
+ *
+ * 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() {
+    }
+
+    /**
+     * Determines if a given character is a valid arithmetic operator.
+     *
+     * @param token the character to check
+     * @return true if the character is an operator, false otherwise
+     */
+    public static boolean isOperator(char token) {
+        return token == '+' || token == '-' || token == '/' || token == '*' || token == '^';
+    }
+
+    /**
+     * Converts a valid prefix expression to an infix expression.
+     *
+     * @param prefix the prefix expression to convert
+     * @return the equivalent infix expression
+     * @throws NullPointerException     if the prefix expression is null
+     */
+    public static String getPrefixToInfix(String prefix) {
+        if (prefix == null) {
+            throw new NullPointerException("Null prefix expression");
+        }
+        if (prefix.isEmpty()) {
+            return "";
+        }
+
+        Stack<String> stack = new Stack<>();
+
+        // Iterate over the prefix expression from right to left
+        for (int i = prefix.length() - 1; i >= 0; i--) {
+            char token = prefix.charAt(i);
+
+            if (isOperator(token)) {
+                // Pop two operands from stack
+                String operandA = stack.pop();
+                String operandB = stack.pop();
+
+                // Form the infix expression with parentheses
+                String infix = "(" + operandA + token + operandB + ")";
+
+                // Push the resulting infix expression back onto the stack
+                stack.push(infix);
+            } else {
+                // Push operand onto stack
+                stack.push(Character.toString(token));
+            }
+        }
+
+        if (stack.size() != 1) {
+            throw new ArithmeticException("Malformed prefix expression");
+        }
+
+        return stack.pop(); // final element on the stack is the full infix expression
+    }
+}