Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation 

问题：

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

思路：

　　栈的经典应用

我的代码：

public class Solution {

    public int evalRPN(String[] tokens) {

        if(tokens == null || tokens.length == 0)    return 0;

        Stack<Integer> stack = new Stack<Integer>();

        for(int i = 0; i < tokens.length; i++)

        {

            String str = tokens[i];

            if(isOperator(str))

            {

                int one = stack.pop();

                int two = stack.pop();

                stack.push(getResult(two, one, str));

            }

            else

            {

                stack.push(Integer.valueOf(str));

            }

        }

        return stack.isEmpty() ? 0 : stack.pop();

    }

    public int getResult(int one, int two, String str)

    {

        switch(str)

        {

            case "+": return one + two; 

            case "-": return one - two; 

            case "*": return one * two; 

            case "/": return one / two;

        }

        return 0;

    }

    public boolean isOperator(String str)

    {

        if(str.equals("+") || str.equals("-") || str.equals("*") || str.equals("/")) return true;

        return false;

    }

}

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