lintcode 中等题：Evaluate Reverse Polish notation逆波兰表达式求值

public class Solution {
    /**
     * @param tokens The Reverse Polish Notation
     * @return the value
     */
    public int evalRPN(String[] tokens) {
        // Write your code here
        if(tokens == null)
            return 0;
        if(tokens.length == 1)
            return Integer.valueOf(tokens[0]);
        Stack<Integer> stack = new Stack<Integer>();
        for(int i =0;i< tokens.length ;i++){
            String str = tokens[i];
            
            if(str.matches("\\d+") == true || 
                str.substring(0,1).equals("-")&&str.substring(1,str.length()).matches("\\d+") == true){
                int num = Integer.valueOf(str);
                stack.push(num);
            }else{
                if(stack.empty()){
                    System.out.println("the stack is empty");
                    return -1;
                }
                int num2 = stack.pop();
                int num1 = stack.pop();
                
                int res = calculate(num1,num2,str);
                stack.push(res);
            }
        }
        return stack.pop();
    }
    public int calculate(int num1,int num2,String symbol){
        if(symbol.equals("+"))
            return num1+ num2;
        if(symbol.equals("-"))
            return num1 - num2;
        if(symbol.equals("*"))
            return num1*num2;
        if(symbol.equals("/")){
            if(num2!=0){
                return num1/num2;
            }else{
                return Integer.MAX_VALUE;
            }
        }else{
            return Integer.MAX_VALUE;
        }
    }
    
}

class Solution:
    # @param {string[]} tokens The Reverse Polish Notation
    # @return {int} the value
    def evalRPN(self, tokens):
        # Write your code here
        if tokens == None:
            return 0
        if len(tokens) == 1:
            return int(tokens[0])
        stack = []
        # print 13//3
        # print 13/3
        # print 6/(135) 0
        # print 6/(-135) -1
        for tk in tokens:
            if tk.isdigit() or tk[0] == ‘-‘ and tk[1:].isdigit():
                stack.append(int(tk))
            else:
                num2 = stack.pop()
                num1 = stack.pop()
                num = self.calculate(num1,num2,tk)
                stack.append(num)
            if(len(tokens) == 13):
                print stack 
        return stack.pop()
        
    def calculate(self,num1,num2,sign):
        if sign ==‘-‘:
            return num1 - num2
        elif sign == ‘+‘:
            return num1 + num2
        elif sign == ‘*‘:
            return num1 * num2
        elif sign == ‘/‘:
            if num2!=0:
                return -num1//num2
            else:
                return 0
        else:
            return 0