From bdd0b6131d41f21d9eba1144df6167b266237055 Mon Sep 17 00:00:00 2001
From: Prasoon Pandey <138553057+Prasoon31Pandey@users.noreply.github.com>
Date: Sun, 5 Oct 2025 19:24:22 +0530
Subject: [PATCH] Add Infix to Postfix Converter using Stack (Java)

This program converts an infix expression (like A+B*C) into a postfix expression (A B C * +)
using the Stack data structure. It follows operator precedence and associativity rules to
ensure the correct order of operations.

Example:
Input:  A+B*C
Output: ABC*+

Time Complexity: O(n)
Space Complexity: O(n)
---
 InfixToPostfix.java | 72 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 InfixToPostfix.java

diff --git a/InfixToPostfix.java b/InfixToPostfix.java
new file mode 100644
index 00000000..ab1f8535
--- /dev/null
+++ b/InfixToPostfix.java
@@ -0,0 +1,72 @@
+import java.util.Stack;
+
+/**
+ * This class converts an infix expression (e.g. A+B*C)
+ * into a postfix expression (e.g. A B C * +)
+ * using the Stack data structure.
+ *
+ * Time Complexity: O(n)
+ * Space Complexity: O(n)
+ */
+public class InfixToPostfix {
+
+    private static boolean isOperand(char c) {
+        return Character.isLetterOrDigit(c);
+    }
+
+    private static int precedence(char op) {
+        switch (op) {
+            case '^': return 3;
+            case '*': case '/': return 2;
+            case '+': case '-': return 1;
+            default: return -1;
+        }
+    }
+
+    public static String convert(String exp) {
+        StringBuilder result = new StringBuilder();
+        Stack<Character> stack = new Stack<>();
+
+        for (int i = 0; i < exp.length(); i++) {
+            char c = exp.charAt(i);
+
+            // Operand → add to result
+            if (isOperand(c)) {
+                result.append(c);
+            }
+            // Opening parenthesis
+            else if (c == '(') {
+                stack.push(c);
+            }
+            // Closing parenthesis
+            else if (c == ')') {
+                while (!stack.isEmpty() && stack.peek() != '(') {
+                    result.append(stack.pop());
+                }
+                if (!stack.isEmpty() && stack.peek() == '(') {
+                    stack.pop();
+                }
+            }
+            // Operator
+            else {
+                while (!stack.isEmpty() && precedence(stack.peek()) >= precedence(c)) {
+                    result.append(stack.pop());
+                }
+                stack.push(c);
+            }
+        }
+
+        // Pop remaining operators
+        while (!stack.isEmpty()) {
+            result.append(stack.pop());
+        }
+
+        return result.toString();
+    }
+
+    public static void main(String[] args) {
+        String exp = "A+B*C";
+        System.out.println("Infix: " + exp);
+        System.out.println("Postfix: " + convert(exp));
+    }
+}