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=0;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;
    }
};