Evaluate Reverse Polish Notation

Evaluate 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


c++版本：

class Solution{

public:

    int evalRPN(vector<string> &tokens){

        if(tokens.size()==1)

            return stoi(*tokens.begin());

        stack<int> numbers;

        vector<string>::iterator it=tokens.begin();

        int op1=0;

        int op2=0;

        while(it!=tokens.end()){

                if(*it=="+"||*it=="-"||*it=="*"||*it=="/"){

                    op2=numbers.top();

                    numbers.pop();

                    op1=numbers.top();

                    numbers.pop();

                    if(*it=="+")  numbers.push(op1+op2);

                    else if(*it=="-") numbers.push(op1-op2);

                    else if(*it=="*") numbers.push(op1*op2);

                    else              numbers.push(op1/op2);

                }

                else

                    numbers.push(stoi(*it));

                    it++;

            }

                return numbers.top();}



};

　　Java版本：

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;

    }

}