60 Permutation Sequence

这道题纯考数学，懒得打字解释了 方法如下

from math import factorial 
class Solution:
    # @param {integer} n
    # @param {integer} k
    # @return {string}
    def getPermutation(self, n, k):
        digits = [i for i in range(1,n + 1)]
        ans = []
        k -= 1
        while n > 0:
            ind = k / factorial(n-1)
            d = digits[ind]
            ans.append(str(d))
            digits.remove(d)
            k %= factorial(n-1)
            n -= 1
        return "".join(ans)