58扩展. 左旋转字符串

class Solution:
  def LeftRotateString(self, s, n):
      # write code here
      if not s or n == 0:
          return s
      n = n % len(s)
      s = list(s)
      first = self.reverse(s[:n])
      last = self.reverse(s[n:])
      return ''.join(self.reverse(first + last))
  def reverse(self, s):
      head = 0
      end = len(s) - 1
      while head < end:
          s[head], s[end] = s[end], s[head]
          head += 1
          end -= 1
      return s