LeetCode - Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

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

Solution:

public class Solution {
    public int evalRPN(String[] tokens) {
        int res = 0;
        if(tokens.length == 0) return res;
        
        // build a stack for this kind of problem
        String operators = "+-*/";
        Stack<string> stack = new Stack<string>();
        
        for(String s : tokens) {
            // if s is not an valid operator, then s is a num, push in stack
            if(!operators.contains(s)) {
                stack.push(s);
            } 
            // if s is an valid operator, pop two nums, calculate, push back
            else {
                int a = Integer.valueOf(stack.pop());
                int b = Integer.valueOf(stack.pop());
                
                // caculate
                switch(s) {
                    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;
                }
            }
        }
        
        // after loop, pop the last num as res
        res = Integer.valueOf(stack.pop());
        
        return res;
    }
}

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

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