LeetCode – Evaluate Reverse Polish Notation

The problem:

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

1. Naive Approach

This problem is simple. After understanding the problem, we should quickly realize that this problem can be solved by using a stack. We can loop through each element in the given array. When it is a number, push it to the stack. When it is an operator, pop two numbers from the stack, do the calculation, and push back the result.

The following is the code. It runs great by feeding a small test. However, this code contains compilation errors in leetcode. Why?

public class Test {
 
	public static void main(String[] args) throws IOException {
		String[] tokens = new String[] { "2", "1", "+", "3", "*" };
		System.out.println(evalRPN(tokens));
	}
 
	public static int evalRPN(String[] tokens) {
		int returnValue = 0;
		String operators = "+-*/";
 
		Stack<String> stack = new Stack<String>();
 
		for (String t : tokens) {
			if (!operators.contains(t)) {
				stack.push(t);
			} else {
				int a = Integer.valueOf(stack.pop());
				int b = Integer.valueOf(stack.pop());
				switch (t) {
				case "+":
					stack.push(String.valueOf(a + b));
					break;
				case "-":
					stack.push(String.valueOf(b - a));
					break;
				case "*":
					stack.push(String.valueOf(a * b));
					break;
				case "/":
					stack.push(String.valueOf(b / a));
					break;
				}
			}
		}
 
		returnValue = Integer.valueOf(stack.pop());
 
		return returnValue;
	}
}

The problem is that switch string statement is only available from JDK 1.7. Leetcode apparently use versions below that.

2. Accepted Solution

If you want to use switch statement, you can convert the above by using the following code which use the index of a string “+-*/”.

public class Solution {
    public int evalRPN(String[] tokens) {
 
        int returnValue = 0;
 
        String operators = "+-*/";
 
        Stack<String> stack = new Stack<String>();
 
        for(String t : tokens){
            if(!operators.contains(t)){
                stack.push(t);
            }else{
                int a = Integer.valueOf(stack.pop());
                int b = Integer.valueOf(stack.pop());
                int index = operators.indexOf(t);
                switch(index){
                    case 0:
                        stack.push(String.valueOf(a+b));
                        break;
                    case 1:
                        stack.push(String.valueOf(b-a));
                        break;
                    case 2:
                        stack.push(String.valueOf(a*b));
                        break;
                    case 3:
                        stack.push(String.valueOf(b/a));
                        break;
                }
            }
        }
 
        returnValue = Integer.valueOf(stack.pop());
 
        return returnValue;
 
    }
}

http://www.programcreek.com/2012/12/leetcode-evaluate-reverse-polish-notation/