package problems.medium;
import java.util.Stack;
/**
* Created by sherxon on 2/6/17.
*/
/**
* Evaluate the value of an arithmetic expression in Reverse Polish Notation.
* Valid operators are +, -, *, /. Each operand may be an integer or another expression.
* Some examples:
* ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
* ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
*/
public class EvaluateReversePolishNotation {
public static void main(String[] args) {
System.out.println(evalRPN(new String[]{"2"}));
}
/**
* To solve this problem, I used stack. if the array element is operator, two elements from stack is popped
* and do operation according to operator and push back to stack. In this way we calculate the whole expression
* The last stack element is result of expression
*/
static int evalRPN(String[] tokens) {
if (tokens.length == 0) return 0;
Stack<String> stack = new Stack<>();
for (String token : tokens) {
if (token.length() == 1 && !Character.isDigit(token.charAt(0))) {
int f = Integer.parseInt(stack.pop());
int s = Integer.parseInt(stack.pop());
if (token.equals("+")) stack.push(String.valueOf(f + s));
else if (token.equals("/")) stack.push(String.valueOf(s / f));
else if (token.equals("*")) stack.push(String.valueOf(s * f));
else stack.push(String.valueOf(s - f));
} else stack.push(token);
}
return Integer.parseInt(stack.pop());
}
}