一文看懂字符串的加减乘除

/**
 * 字符串的加减乘除
*/
int calculate(string s) {
	stack<int> stk;
	int num = 0;
	// 记录num前符号，初始化为'+'
	char sign = '+';
	for (int i = 0; i < s.size(); i++) {
		char c = s[i];
		if (isdigit(c)) {
			num = num * 10 + (c - '0');
		}
		if (!isdigit(c) && (c != ' ') || (i == s.size() - 1)) {
			switch (sign)
			{
			case '+':
				stk.push(num);
				break;
			case '-':
				stk.push(-num);
				break;
			case '*':
				pre = stk.top();
				stk.pop();
				stk.push(pre * num);
				break;
			case '/':
				pre = stk.top();
				stk.pop();
				stk.push(pre / num);
				break;
			default:
				break;
			}
			// 更新符号为当前符号，数字清零
			sign = c;
			num = 0;
		}
	}
	int res = 0;
	while(!stk.empty()) {
		res += stk.top();
		stk.pop();
	}
	return res;
}```