InfixtoPostfix.java

import java.util.Stack; 

class Test 
{ 
	
	static int Prec(char ch) 
	{ 
		switch (ch) 
		{ 
		case '+': 
		case '-': 
			return 1; 
	
		case '*': 
		case '/': 
			return 2; 
	
		case '^': 
			return 3; 
		} 
		return -1; 
	} 
	

	static String infixToPostfix(String exp) 
	{ 
		
		String result = new String(""); 
		
		
		Stack<Character> stack = new Stack<>(); 
		
		for (int i = 0; i<exp.length(); ++i) 
		{ 
			char c = exp.charAt(i); 
			
		
			if (Character.isLetterOrDigit(c)) 
				result += c; 
			
			
			else if (c == '(') 
				stack.push(c); 
		
			else if (c == ')') 
			{ 
				while (!stack.isEmpty() && stack.peek() != '(') 
					result += stack.pop(); 
				
				if (!stack.isEmpty() && stack.peek() != '(') 
					return "Invalid Expression"; // invalid expression				 
				else
					stack.pop(); 
			} 
			else // an operator is encountered 
			{ 
				while (!stack.isEmpty() && Prec(c) <= Prec(stack.peek())){ 
					if(stack.peek() == '(') 
						return "Invalid Expression"; 
					result += stack.pop(); 
			} 
				stack.push(c); 
			} 
	
		} 
	
	
		while (!stack.isEmpty()){ 
			if(stack.peek() == '(') 
				return "Invalid Expression"; 
			result += stack.pop(); 
		} 
		return result; 
	} 
	
	public static void main(String[] args) 
	{ 
		String exp = "a+b*(c^d-e)^(f+g*h)-i"; 
		System.out.println(infixToPostfix(exp)); 
	} 
}