38. Count and Say

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 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 sequence.

Note: The sequence of integers will be represented as a string.

class Solution(object):
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        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

re.sub（pattern,repl,string,count=0,flags=0）实现正则替换

(.)\1*
　　.匹配任意一个字符
　　（）组成一个group
　　\1和第一个group一样

lambda用来创建匿名函数

m.group()返回整个匹配
m.group(1)返回第一个匹配的group

class Solution(object):
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        s = '1'
        for i in range(n-1):
            count = 1
            temp = []
            for index in range(1, len(s)):
                if s[index] == s[index-1]:
                    count += 1
                else:
                    temp.append(str(count))
                    temp.append(s[index-1])
                    count = 1
            temp.append(str(count))
            temp.append(s[-1])
            s = ''.join(temp)
        return s

 

for index in range(n)是从0开始循环的。