[算法]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. For example:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

1. Naive Approach

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.

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)) { //push to stack if it is a number

				stack.push(t);

			} else {//pop numbers from stack if it is an operator

				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;

	}

}

or

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;


    }

}