227. Basic Calculator II

class Solution {
public:
int calculate(string s) {
istringstream is('+'+s+'+');
char op;
int result=0,n,prevalue=0;
while (is>>op)
{
if (op=='+'||op=='-')
{
result+=prevalue;
is>>prevalue;
prevalue*=op=='+'?1:-1;
}
else
{
is>>n;
prevalue = op=='*'?prevalue*n:prevalue/n;
}
}
return result;
}
};