Evaluate the value of an arithmetic expression in .

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
     
       &tokens) {        int ans=0;        stack
      
        s;        int a,b,c;        for(int i=0;i