evaluate-reverse-polish-notation——栈

Evaluate the value of an arithmetic expression in 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

将字符串序列转化为算式并计算，且符号在数字之后

 class Solution {
 public:
     int evalRPN(vector<string> &tokens) {
         stack<int> s;
         for(int i=;i<tokens.size();i++){
             string cur = tokens[i];
             int temp = atoi(cur.c_str());
             if(cur=="+"||cur=="-"||cur=="*"||cur=="/"){
                 int left,right;
                 if(!s.empty()){
                     right=s.top();
                     s.pop();
                 }
                 if(!s.empty()){
                     left=s.top();
                     s.pop();
                 }
                 if(cur=="+")
                     temp=left+right;
                 else if(cur=="-")
                     temp=left-right;
                 else if(cur=="*")
                     temp=left*right;
                 else if(cur=="/")
                     temp=left/right;

             }
             s.push(temp);
         }
         int res = s.top();
         return res;
     }
 };