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
        
          &tokens){        if(tokens.size()==1)            return stoi(*tokens.begin());        stack
         
           numbers;        vector
          
           ::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
        
          stack = new Stack
         
          ();        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;    }}