Create infix-to-postfix.md

Java/Stack/infix-to-postfix.md

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+# Infix Notation
+
+Infix is the day to day notation that we use of format A + B type. The general form can be classified as (a op b) where a and b are operands(variables) and op is Operator.
+
+# Postfix Notation
+
+Postfix is notation that compiler uses/converts to while reading left to right and is of format AB+ type. The general form can be classified as (ab op) where a and b are operands(variables) and op is Operator.
+
+## Code
+
+``` 
+import java.util.Stack;
+
+public class InfixtoPostfix {
+	public static void main(String[] args) {
+		String exp = "a+b*(c^d-e)^(f+g*h)-i";
+		System.out.println(infixToPostfix(exp));
+	}
+	public static int Precidense(char ch) {
+		switch (ch){
+      case '+':
+      case '-':
+			  return 1;
+
+      case '*':
+      case '/':
+        return 2;
+
+      case '^':
+        return 3;
+		}
+		return -1;
+	}
+
+	public static String infixToPostfix(String exp) {
+		String result = "";
+		Stack<Character> stack = new Stack<>();
+
+		for (char c:exp.toCharArray()) {
+			if (Character.isLetterOrDigit(c))
+				result += c;
+			else if (c == '(')
+				stack.push(c);
+			else if (c == ')') {
+				while (!stack.isEmpty() && stack.peek() != '(')
+					result += stack.pop();
+					stack.pop();
+			}
+			else {// an operator is encountered
+				while (!stack.isEmpty() && Precidense(c) <= Precidense(stack.peek()))
+					result += stack.pop();	
+				stack.push(c);
+			}
+
+		}
+		while (!stack.isEmpty()){
+			if(stack.peek() == '(')
+				return "Invalid Expression";
+			result += stack.pop();
+		}
+		return result;
+	}
+}
+```