leetcode Evaluate Reverse Polish Notation(计算逆波兰表达式)

题目要求：

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

我的c++代码:

int evalRPN(vector<string> &tokens)
{
if(tokens.size() == 0)
return 0;
string op = "+-*/";
stack<int> st;
for(size_t i = 0; i < tokens.size(); ++i)
{
int flag = op.find(tokens[i]);
if(flag != string::npos)
{
if(st.size() < 2)
return -1;
int a = st.top();
st.pop();
int b = st.top();
st.pop();
if(flag == 0)
st.push(a + b);
if(flag == 1)
st.push(b - a);
if(flag == 2)
st.push(a * b);
if(flag == 3)
st.push(b / a);
}
else
st.push(atoi(tokens[i].c_str()));
}
return st.top();
}