leetcode 日经贴，Cpp code -Evaluate Reverse Polish Notation

class Solution {
public:
int a2i(const string &s) {
int n = 0, neg = 1;
for (int i = 0; i < s.length(); ++i) {
if (s[i] == '-' && i == 0) {
neg = -1;
} else {
n = n * 10 + int(s[i] - '0');
}
}
return neg * n;
}
int evalRPN(vector<string>& tokens) {
stack<int> st;
for (int i = 0; i < tokens.size(); ++i) {
if (tokens[i].length() > 1 || (tokens[i][0] >= '0' && tokens[i][0] <= '9')) {
st.push(a2i(tokens[i]));
} else {
int b = st.top();
st.pop();
int a = st.top();
st.pop();
int c = 0;
if (tokens[i] == "+") {
c = a + b;
} else if (tokens[i] == "-") {
c = a - b;
} else if (tokens[i] == "*") {
c = a * b;
} else if (tokens[i] == "/") {
c = a / b;
}
st.push(c);
}
}
return st.top();
}
};