leetcode 31-40 easy

38、Count and Say

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     111221

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

C+

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

python

Solution 1 ... using a regular expression
def countAndSay(self, n):
    s = ''
    for _ in range(n - 1):
        s = re.sub(r'(.)\1*', lambda m: str(len(m.group(0))) + m.group(1), s)
    return s
Solution 2 ... using a regular expression
def countAndSay(self, n):
    s = ''
    for _ in range(n - 1):
        s = ''.join(str(len(group)) + digit
                    for group, digit in re.findall(r'((.)\2*)', s))
    return s
Solution 3 ... using groupby
def countAndSay(self, n):
    s = ''
    for _ in range(n - 1):
        s = ''.join(str(len(list(group))) + digit
                    for digit, group in itertools.groupby(s))
    return s