[BAEL-7501] Infix to Postfix (#15819)

Co-authored-by: Bhaskar <bhaskar.dastidar@freshworks.com>

core-java-modules/core-java-lang-operators-2/src/main/java/com/baeldung/infixpostfix/InfixToPostFixExpressionConversion.java

@@ -0,0 +1,67 @@
+package com.baeldung.infixpostfix;
+
+import java.util.Stack;
+
+public class InfixToPostFixExpressionConversion {
+
+    private int getPrecedenceScore(char ch) {
+        switch (ch) {
+        case '^':
+            return 3;
+
+        case '*':
+        case '/':
+            return 2;
+
+        case '+':
+        case '-':
+            return 1;
+        }
+        return -1;
+    }
+
+    private boolean isOperand(char ch) {
+        return (ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9');
+    }
+
+    private char associativity(char ch) {
+        if (ch == '^')
+            return 'R';
+        return 'L';
+    }
+
+    public String infixToPostfix(String infix) {
+        StringBuilder result = new StringBuilder();
+        Stack<Character> stack = new Stack<>();
+
+        for (int i = 0; i < infix.length(); i++) {
+            char ch = infix.charAt(i);
+
+            if (isOperand(ch)) {
+                result.append(ch);
+            } else if (ch == '(') {
+                stack.push(ch);
+            } else if (ch == ')') {
+                while (!stack.isEmpty() && stack.peek() != '(') {
+                    result.append(stack.pop());
+                }
+                stack.pop();
+            } else {
+                while (!stack.isEmpty() && (operatorPrecedenceCondition(infix, i, stack))) {
+                    result.append(stack.pop());
+                }
+                stack.push(ch);
+            }
+        }
+
+        while (!stack.isEmpty()) {
+            result.append(stack.pop());
+        }
+
+        return result.toString();
+    }
+
+    private boolean operatorPrecedenceCondition(String infix, int i, Stack<Character> stack) {
+        return getPrecedenceScore(infix.charAt(i)) < getPrecedenceScore(stack.peek()) || getPrecedenceScore(infix.charAt(i)) == getPrecedenceScore(stack.peek()) && associativity(infix.charAt(i)) == 'L';
+    }
+}

core-java-modules/core-java-lang-operators-2/src/test/java/com/baeldung/infixpostfix/InfixToPostfixExpressionUnitTest.java

@@ -0,0 +1,37 @@
+package com.baeldung.infixpostfix;
+
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+
+public class InfixToPostfixExpressionUnitTest {
+
+    @Test
+    public void givenSimpleOp_whenNoParenthesis_thenProduceValidPostfix() {
+        String infix = "a+b*c-d";
+        String postfix = "abc*+d-";
+
+        InfixToPostFixExpressionConversion obj = new InfixToPostFixExpressionConversion();
+
+        Assertions.assertEquals(postfix, obj.infixToPostfix(infix));
+    }
+
+    @Test
+    public void givenSimpleOp_whenWithParenthesis_thenProduceValidPostfix() {
+        String infix = "(a+b)*(c-d)";
+        String postfix = "ab+cd-*";
+
+        InfixToPostFixExpressionConversion obj = new InfixToPostFixExpressionConversion();
+
+        Assertions.assertEquals(postfix, obj.infixToPostfix(infix));
+    }
+
+    @Test
+    public void givenComplexOp_whenInputIsInfix_thenProduceValidPostfix() {
+        String infix = "a^b*(c^d-e)^(f+g*h)-i";
+        String postfix = "ab^cd^e-fgh*+^*i-";
+
+        InfixToPostFixExpressionConversion obj = new InfixToPostFixExpressionConversion();
+
+        Assertions.assertEquals(postfix, obj.infixToPostfix(infix));
+    }
+}