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

class Solution {
public:

bool isFlag(string str) {
if(str.length() != 1)
return false;
if(str[0] == '-' || str[0] == '+' || str[0] == '/' || str[0] == '*')
return true;
return false;
}

int calculate(int left, int right, string flag) {
if(flag[0] == '+')
return left + right;
if(flag[0] == '-')
return left - right;
if(flag[0] == '*')
return left * right;
if(flag[0] == '/')
return left / right;
return 0;
}

int stringToNumber(string str) {
int number = 0;
int flag = 1;
for(int ii = 0; ii < str.length(); ii ++) {
if(ii == 0) {
if(str[0] == '-') {
flag = -1;
continue;
}
else if(str[0] == '+') {
continue;
}
}
number = number * 10 + str[ii] - '0';
}
return number * flag;
}

int evalRPN(vector<string> &tokens) {
if(tokens.size() == 0)
return 0;

stack<int> st;
for(int ii = 0; ii < tokens.size(); ii ++) {
if(isFlag(tokens[ii])) {
int right = st.top();
st.pop();
int left = st.top();
st.pop();
int current = calculate(left, right, tokens[ii]);
st.push(current);
}
else {
st.push(stringToNumber(tokens[ii]));
}
}
return st.top();
}
};