# 基本计算器 II
给你一个字符串表达式 s ，请你实现一个基本计算器来计算并返回它的值。
整数除法仅保留整数部分。
示例 1：
输入：s = "3+2*2"
输出：7
示例 2：
输入：s = " 3/2 "
输出：1
示例 3：
输入：s = " 3+5 / 2 "
输出：5
提示：
1 <= s.length <= 3 * 105s 由整数和算符 ('+', '-', '*', '/') 组成，中间由一些空格隔开s 表示一个 有效表达式- 表达式中的所有整数都是非负整数，且在范围
[0, 231 - 1] 内
- 题目数据保证答案是一个 32-bit 整数
以下错误的选项是？
## aop
### before
```cpp
#include
using namespace std;
```
### after
```cpp
int main()
{
Solution sol;
int res;
string s1 = "3+2*2";
res = sol.calculate(s1);
cout << res;
return 0;
}
```
## 答案
```cpp
class Solution
{
public:
int calculate(string s)
{
int res = 0;
stack my_stack{};
vector my_vec{};
for (auto x : s)
{
if (x != ' ')
my_vec.push_back(x);
}
int start = 0;
int end = 0;
int tmp = 0;
while (start != my_vec.size())
{
tmp = 0;
for (int i = start; i < my_vec.size(); i++)
{
if (my_vec[i] >= '0')
{
if (i == 0)
{
tmp = my_vec[i] - '0';
}
else if (my_vec[i - 1] == '+' || my_vec[i - 1] == '*' || my_vec[i - 1] == '/')
tmp = my_vec[i] - '0';
else if (my_vec[i - 1] == '-')
tmp = -1 * (my_vec[i] - '0');
else if (my_vec[i - 1] >= '0')
{
if (tmp >= 0)
tmp = tmp * 10 + (my_vec[i] - '0');
else
tmp = tmp * 10 - (my_vec[i] - '0');
}
if (i == my_vec.size() - 1)
end = i;
}
else
{
end = i;
break;
}
}
if (start == 0)
my_stack.push(tmp);
else
{
if (my_vec[start - 1] == '+' || my_vec[start - 1] == '-')
my_stack.push(tmp);
else if (my_vec[start - 1] == '*')
my_stack.top() = my_stack.top() * tmp;
else if (my_vec[start - 1] == '/')
my_stack.top() = my_stack.top() / tmp;
}
start = end + 1;
}
while (!my_stack.empty())
{
res = my_stack.top();
my_stack.pop();
}
return res;
}
};
```
## 选项
### A
```cpp
class Solution
{
public:
int calculate(string s)
{
int ans = 0;
int n = s.size();
char ope = '+';
stack temp;
int d = 0;
int A;
for (int i = 0; i < n; i++)
{
if (s[i] - '0' >= 0 && s[i] - '0' <= 9)
{
d = d * 10 + (s[i] - '0');
}
if ((s[i] < '0' && s[i] != ' ') || i == n - 1)
{
if (ope == '+')
{
temp.push(d);
}
if (ope == '-')
{
temp.push(-d);
}
if (ope == '*')
{
A = temp.top() * d;
temp.pop();
temp.push(A);
}
if (ope == '/')
{
A = temp.top() / d;
temp.pop();
temp.push(A);
}
ope = s[i];
d = 0;
}
}
while (!temp.empty())
{
ans += temp.top();
temp.pop();
}
return ans;
}
};
```
### B
```cpp
class Solution
{
public:
int calculate(string s)
{
stack fuhao;
stack conclude;
int i = 0, len = s.length();
while (i < len)
{
if (s[i] == ' ')
i++;
else if (s[i] >= '0' && s[i] <= '9')
{
long x = s[i] - '0';
i++;
while (i < len && s[i] >= '0' && s[i] <= '9')
{
x = x * 10 + s[i] - '0';
i++;
}
conclude.push(x);
}
else
{
while (!fuhao.empty() && (fuhao.top() == '*' || fuhao.top() == '/' || s[i] == '+' || s[i] == '-'))
{
long x = conclude.top();
conclude.pop();
long y = conclude.top();
conclude.pop();
if (fuhao.top() == '+')
conclude.push(y + x);
else if (fuhao.top() == '-')
conclude.push(y - x);
else if (fuhao.top() == '*')
conclude.push(y * x);
else
conclude.push(y / x);
fuhao.pop();
}
fuhao.push(s[i]);
i++;
}
}
while (!fuhao.empty())
{
long x = conclude.top();
conclude.pop();
long y = conclude.top();
conclude.pop();
if (fuhao.top() == '+')
conclude.push(y + x);
else if (fuhao.top() == '-')
conclude.push(y - x);
else if (fuhao.top() == '*')
conclude.push(y * x);
else
conclude.push(y / x);
fuhao.pop();
}
return conclude.top();
}
};
```
### C
```cpp
class Solution
{
public:
int calculate(string s)
{
vector stk;
char preSign = '+';
int num = 0;
int n = s.length();
for (int i = 0; i < n; ++i)
{
if (isdigit(s[i]))
{
num = num * 10 + int(s[i] - '0');
}
if (!isdigit(s[i]) && s[i] != ' ' || i == n - 1)
{
switch (preSign)
{
case '+':
stk.push_back(num);
break;
case '-':
stk.push_back(-num);
break;
case '*':
stk.back() *= num;
break;
default:
stk.back() /= num;
}
preSign = s[i];
num = 0;
}
}
return accumulate(stk.begin(), stk.end(), 0);
}
};
```