[LeetCode]224. Basic Calculator（模拟，栈）

 1 class Solution {
 2 public:
 3     int calculate(string s) {
 4         int n = s.length();
 5         stack<int> st;
 6         stack<char> op;
 7         int ret = 0;
 8         op.push(‘+‘);
 9         for(int i = 0; i < n; i++) {
10             if(isdigit(s[i])) {
11                 int x = 0;
12                 while(isdigit(s[i])) x = x * 10 + s[i++] - ‘0‘;
13                 i--;
14                 char o = op.top(); op.pop();
15                 if(o == ‘+‘) ret += x;
16                 else ret -= x;
17             }
18             else if(s[i] == ‘+‘ || s[i] == ‘-‘) op.push(s[i]);
19             else if(s[i] == ‘(‘) {
20                 st.push(ret);
21                 op.push(‘+‘);
22                 ret = 0;
23             }
24             else if(s[i] == ‘)‘) {
25                 char o = op.top(); op.pop();
26                 if(o == ‘+‘) ret += st.top();
27                 else ret = st.top() - ret;
28                 st.pop();
29             }
30         }
31         return ret;
32     }
33 };