Reverse String II — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def reverseStr(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: str
        """
        s = list(s)
        for i in xrange(0, len(s), 2*k):
            s[i:i+k] = reversed(s[i:i+k])
        return "".join(s)