[LeetCode]Evaluate Reverse Polish Notation

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

思考：注意负数。

class Solution {
public:
    int stoi(string str)
    {
		bool flag=true;
		if(str[0]=='-') flag=false;
        int ret=0,i;
		i=(flag)?0:1;
        for(;i<str.size();i++)
        {
            ret=ret*10+(str[i]-'0');
        }
        return (flag)?ret:(-ret);
    }
    int evalRPN(vector<string> &tokens) {
        int ans=0;
        stack<int> s;
        int a,b,c;
        for(int i=0;i<tokens.size();i++)
        {
            if(tokens[i]!="+"&&tokens[i]!="-"&&tokens[i]!="*"&&tokens[i]!="/")
            {
                string str=tokens[i];
                int temp=stoi(str);
                s.push(temp);
            }
            else
            {
                a=s.top();
                s.pop();
                b=s.top();
                s.pop();
                if(tokens[i]=="+") c=b+a;
                if(tokens[i]=="-") c=b-a;
                if(tokens[i]=="*") c=b*a;
                if(tokens[i]=="/") c=b/a;
                s.push(c);
            }
        }
        ans=s.top();
        return ans;
    }
};