[LeetCode 解题报告]227. Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

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;
        char sign = '+';
        stack<long> st;
        
        for (int i = 0; i < s.size(); i ++) {
            char ch = s[i];
            if (ch >= '0')
                num = num*10 + ch - '0';
            if ((ch < '0' && ch != ' ') || i == s.size()-1) {
                if (sign == '+')
                    st.push(num);
                if (sign == '-')
                    st.push(-num);
                if (sign == '*' || sign == '/') {
                    int tmp = (sign == '*') ? num*st.top() : st.top()/num;
                    st.pop();
                    st.push(tmp);
                }
                sign = s[i];
                num = 0;
            }
        }
        
        while (!st.empty()) {
            res += st.top();
            st.pop();
        }
        return res;
    }
};