# 字符串转换整数 (atoi)
以下错误的选项是？
## aop
### before
```cpp
#include <bits/stdc++.h>
using namespace std;
```
### after
```cpp
int main()
{
    Solution sol;
    cout << sol.myAtoi("-42") << endl;
    return 0;
}
```

## 答案
```cpp
class Solution
{
public:
    int myAtoi(string s)
    {
        int index = 0;

        while (index < s.length() && s[index] == ' ')
            index++;

        int sign = 1;
        if (index < s.length() && (s[index] == '+' || s[index] == '-'))
        {
            if (s[index] == '-')
                sign = -1;
            index++;
        }

        int val = 0;
        while (index < s.length() && (s[index] >= '0' && s[index] <= '9'))
        {
            if (val > (2147483647 - (s[index] - '0')) / 10)
            {
                if (sign == -1)
                    return -2147483648;
                else
                    return 2147483647;
            }

            val = val * 10 + (s[index] - '0');
            index++;
        }

        return val;
    }
};
```
## 选项

### A
```cpp
class Solution
{
public:
    int myAtoi(string str)
    {
        int d = 0;
        istringstream is(str);
        is >> d;
        return d;
    }
};
```

### B
```cpp
class Solution
{
public:
    int myAtoi(string str)
    {
        int i = 0;
        while (' ' == str[i])
            ++i; 
        long flag = 1;
        if ('+' == str[i])
        {
            flag = 1;
            return fun(str, ++i, flag);
        }
        else if ('-' == str[i])
        {
            flag = -1;
            return fun(str, ++i, flag);
        }
        else if (str[i] - '0' >= 0 && str[i] - '0' <= 9)
        {
            flag = 1;
            return fun(str, i, flag);
        }
        else
        {
            return 0;
        }
    }
    int fun(string str, int i, const long flag) 
    {
        long res = 0;
        while (str[i] - '0' >= 0 && str[i] - '0' <= 9)
        {
            res = res * 10 + (str[i++] - '0');
            if (res * flag > pow(2, 31) - 1)
                return INT_MAX;
            if (res * flag < -pow(2, 31))
                return INT_MIN;
        }
        res = res * flag;
        return int(res);
    }
};
```

### C
```cpp
class Solution
{
public:
    int myAtoi(string str)
    {
        int max = INT_MAX / 10;
        int min = INT_MIN / 10;
        int max_f = INT_MAX % 10;
        int min_f = INT_MIN % 10;
        int len = str.length();
        bool flag = false;
        int sum = 0;
        int symbol = 1;
        for (int i = 0; i < len; i++)
        {
            char ch = str[i];
            if (!flag)
            {
                if (ch == ' ')
                    continue;
                else if (ch == '+')
                    flag = true;
                else if (ch == '-')
                {
                    flag = true;
                    symbol = -1;
                }
                else if (ch <= '9' && ch >= '0')
                    flag = true;
                else
                    break;
            }
            else if (ch > '9' || ch < '0')
                break;
            if (ch <= '9' && ch >= '0')
            {
                int pos = symbol * (ch - '0');
                if (sum > max || (sum == max && pos >= max_f))
                    return INT_MAX;
                if (sum < min || (sum == min && pos <= min_f))
                    return INT_MIN;
                sum = sum * 10 + pos;
            }
        }
        return sum;
    }
};
```