[LeetCode 解题报告]224. Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

class Solution {
public:
    int calculate(string s) {
        if (s.empty())
            return 0;
        
        long res = 0, num = 0;
        int sign = 1;
        stack<long> st;
        
        for (int i = 0; i < s.size(); i ++) {
            char ch = s[i];
            if (ch >= '0')
                num = num*10 + ch - '0';
            else if (ch == '+' || ch == '-') {
                res += sign*num;
                num = 0;
                sign = (ch == '+') ? 1 : -1;
            }
            else if (ch == '(') {
                st.push(res);
                st.push(sign);
                res = 0;
                sign = 1;
            }
            else if (ch == ')') {
                res += sign*num;
                num = 0;
                res *= st.top();
                st.pop();
                res += st.top();
                st.pop();
            }
        }
        res += sign * num;
        return res;
    }
};