package stack;

public class PostfixEval {
	public static void main(String[] args) {
		String postfix = "700 3 47 + 6 * - 4 /";
		System.out.println("Input string: " + postfix);
		int answer = evaluate(postfix);
		System.out.println("Answer: " + answer);
	}

	private static int evaluate(String postfix) {
		int A, B;
		LinkedStack<Integer> s = new LinkedStack<>();
		boolean digitPreviously = false;
		for (int i = 0; i < postfix.length(); i++) {
			char ch = postfix.charAt(i);  
			if (Character.isDigit(ch)) {
				if (digitPreviously == true) {
					int tmp = s.pop();
					tmp = 10*tmp + (ch-'0');
					s.push(tmp);
				} else s.push(ch - '0');
				digitPreviously = true;  
			} else if (isOperator(ch)) {
				A = s.pop();
				B = s.pop();
				int val = operation(A, B, ch);
				s.push(val);
				digitPreviously = false;
			} else digitPreviously = false;  
		}
		return s.pop();
	}
 
	private static int operation(int a, int b, char ch) {
		int val = 0;
		switch (ch) {
			case '*':
				val = b * a;
				break;
			case '/':
				val = b / a;
				break;
			case '+':
				val = b + a;
				break;
			case '-':
				val = b - a;
				break;
		}
		return val;
	}

	private static boolean isOperator(char ch) {
		return ch == '+' || ch == '-' || ch == '*' || ch == '/';
	}
} // 코드 6-9