Leetcode#150 Evaluate Reverse Polish Notation

 1 int toInteger(string &s) {
 2   int res = 0;
 3   bool negative = s[0] == ‘-‘ ? true : false;
 4         
 5   for (int i = negative ? 1 : 0; i < s.length(); i++) {
 6     res *= 10;
 7     res += s[i] - ‘0‘;
 8   }
 9   return negative ? -res : res;
10 }
11     
12 int compute(int a, int b, string sign) {
13   switch (sign[0]) {
14   case ‘+‘:
15     return a + b;
16   case ‘-‘:
17     return a - b;
18   case ‘*‘:
19     return a * b;
20   case ‘/‘:
21     return a / b;
22   default:
23     return -1;
24   }
25 }
26     
27 int evalRPN(vector<string> &tokens) {
28   stack<int> st;
29         
30   for (auto t : tokens) {
31     if (t.length() > 1 || t[0] >= ‘0‘ && t[0] <= ‘9‘)
32       st.push(toInteger(t));
33     else {
34       int b = st.top();
35       st.pop();
36       int a = st.top();
37       st.pop();
38       st.push(compute(a, b, t));
39     }
40   }
41         
42   return st.top();
43 }