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# Count and Say

## C++

### 递归

```
class Solution
{
  public:
    string countAndSay(int n)
    {
        if (n == 1)
            return "1";
        else if (n == 2)
            return "11";
        else
        {
            string temp = countAndSay(n - 1);
            string result = "";
            int count = 1;
            int n = temp.size();
            char ch = temp[n - 1];
            for (int i = n - 2; i >= 0; i--)
            {
                if (temp[i] != ch)
                {
                    result = to_string(count) + ch + result;
                    count = 1;
                    ch = temp[i];
                }
                else
                {
                    count++;
                }
            }
            result = to_string(count) + ch + result;
            return result;
        }
    }
};
```

### faster

```
class Solution
{
  public:
    string countAndSay(int n)
    {
        if (n == 0)
            return "";
        string result = "1";
        while (--n)
        {
            string temp = "";
            for (int i = 0; i < result.size(); i++)
            {
                int count = 1;
                while (i + 1 < result.size() && (result[i] == result[i + 1]))
                {
                    count++;
                    i++;
                }
                temp += to_string(count) + result[i];
            }
            result = temp;
        }
        return result;
    }
};
```

### stringstream

* 头文件：sstream

```
class Solution
{
  public:
    string countAndSay(int n)
    {
        string result = "1";
        for (int i = 1; i < n; ++i)
        {
            stringstream next;
            char u;
            int count = 0;
            for (const char &c : result)
            {
                if (count == 0)
                {
                    u = c, count = 1;
                }
                else if (u != c)
                {
                    next << count << u;
                    u = c, count = 1;
                }
                else
                {
                    count++;
                }
            }
            if (count != 0)
            {
                next << count << u;
            }
            result = next.str();
        }
        return result;
    }
};
```

## python

### 使用正则表达式1

```
def countAndSay(self, n):
    s = '1'
    for _ in range(n - 1):
        s = re.sub(r'(.)\1*', lambda m: str(len(m.group(0))) + m.group(1), s)
    return s
```

### 使用正则表达式2

```
def countAndSay(self, n):
    s = '1'
    for _ in range(n - 1):
        s = ''.join(str(len(group)) + digit
                    for group, digit in re.findall(r'((.)\2*)', s))
    return s
```

### 使用groupby

```
def countAndSay(self, n):
    s = '1'
    for _ in range(n - 1):
        s = ''.join(str(len(list(group))) + digit
                    for digit, group in itertools.groupby(s))
    return s
```