feat: 중위 연산식을 후위 연산식으로 바꾸는 converter 구현

Author: wonu606

calculator/src/main/java/com/wonu606/calculator/converter/Converter.java

@@ -0,0 +1,6 @@
+package com.wonu606.calculator.converter;
+
+public interface Converter<T, R> {
+
+    R convert(T input);
+}

calculator/src/main/java/com/wonu606/calculator/converter/InfixToPostfixConverter.java

@@ -0,0 +1,56 @@
+package com.wonu606.calculator.converter;
+
+import com.wonu606.calculator.model.Operator;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+import java.util.Objects;
+
+public class InfixToPostfixConverter implements Converter<String, List<String>> {
+
+    @Override
+    public List<String> convert(String infixExpression) {
+        Deque<String> operatorStack = new ArrayDeque<>();
+        List<String> postfixExpression = new ArrayList<>();
+
+        String[] tokens = infixExpression.split("\\s");
+
+        for (String token : tokens) {
+            processTokenIntoPostfix(operatorStack, postfixExpression, token);
+        }
+
+        while (!operatorStack.isEmpty()) {
+            postfixExpression.add(operatorStack.pop());
+        }
+        return postfixExpression;
+    }
+
+    private void processTokenIntoPostfix(
+            Deque<String> operatorStack, List<String> postfixExpression, String token) {
+        if (isOperator(token)) {
+            popWhileHigherPrecedence(operatorStack, postfixExpression, token);
+            operatorStack.push(token);
+        } else {
+            postfixExpression.add(token);
+        }
+    }
+
+    private void popWhileHigherPrecedence(
+            Deque<String> operatorStack, List<String> postfixExpression, String tokenOperator) {
+        while (!operatorStack.isEmpty()
+                && isHigherOrEqualPrecedence(operatorStack.peek(), tokenOperator)) {
+            postfixExpression.add(operatorStack.pop());
+        }
+    }
+
+    private boolean isHigherOrEqualPrecedence(String peekOperator, String tokenOperator) {
+        int peekPrecedence = Objects.requireNonNull(Operator.get(peekOperator)).precedence;
+        int tokenPrecedence = Objects.requireNonNull(Operator.get(tokenOperator)).precedence;
+        return peekPrecedence <= tokenPrecedence;
+    }
+
+    private boolean isOperator(String token) {
+        return Operator.get(token) != null;
+    }
+}