简单易懂的解法：
把两个s拼起来，然后从第n%len(s)开始取出len(s)个元素，就可以了

class Solution:
    def LeftRotateString(self, s, n):
        # write code here
        if s == None or s == "":
            return ""
        len_s = len(s)
        s = s + s
        n = n%len_s
        return s[n:n+len_s]