[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;

    }

};