227. Basic Calculator II (M)

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.

我的答案：参考了前面224题

class Solution {
public:
    int ret = 0;
    int val1 = 0;
    int val2 = 0;
    bool more = false;
    int sign = 1;
    bool plus;
    int calculate(string s) {
        for (int pos=0; pos<s.size(); ++pos) {
            char c = s[pos];
            // cout << pos << ", " << c << ", ret = " << ret << ", val1 = " << val1 << ", val2 = " << val2 << ", more = " << more << ", plus = " << plus << endl;
            if (isdigit(c)) {
                if (!more) val1 = val1*10 + (c-'0');
                else val2 = val2*10 + (c-'0');
            }
            else if (c == '+' or c == '-') {
                section_end();
                sign = (c == '+')? 1:-1;
                clear();
            }
            else if (c == '*' or c == '/') {
                if (more) {
                    val1 = plus? val1*val2:val1/val2;
                }
                more = true;
                val2 = 0;
                plus = (c == '*')? true:false;
            }
        }
        section_end();
        return ret;
    }
    void clear() {
        val1 = 0;
        val2 = 0;
        more = false;
    }
    void section_end() {
        if (more) {
            val1 = plus? val1*val2:val1/val2;
        }
        ret += val1*sign;
    }
};

Runtime: 8 ms, faster than 94.65% of C++ online submissions for Basic Calculator II.
Memory Usage: 8 MB, less than 94.35% of C++ online submissions for Basic Calculator II.

前面224是有括号，所以需要递归，这边主要是有*乘/除需要在+和-中间优先处理掉。我的方法是加一个plus的bool变量。这边有个小错误是，我只记得在主函数结尾处加了section_end里面的内容，忘记在(c == '+' or c == '-')里面加了