Question:

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

Thinking:

We should use stack and scan the elements of this string array. Whenever we get the operator, we pop element from the stack and push new element to the stack.

Solutioin:

public int evalRPN(String[] tokens) {
    Stack<Integer> stack = new Stack<Integer>();

    for (String s: tokens) {
        if (s.equals("+")) {
            int num1 = stack.pop();
            int num2 = stack.pop();
            stack.push(num1+num2);
        }
        else if (s.equals("-")) {
            int num1 = stack.pop();
            int num2 = stack.pop();
            stack.push(num2-num1);
        }
        else if (s.equals("*")) {
            int num1 = stack.pop();
            int num2 = stack.pop();
            stack.push(num1*num2);
        }
        else if (s.equals("/")) {
            int num1 = stack.pop();
            int num2 = stack.pop();
            stack.push(num2/num1);
        }
        else
            stack.push(Integer.parseInt(s));
    }

    return stack.pop();
}