60. 第k个排列

class Solution:
    def getPermutation(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: str
        """
        ans = ''
        fact = [1] * n
        num = [str(i) for i in range(1, 10)]
        for i in range(1, n):
            fact[i] = fact[i - 1] * i
        k -= 1
        for i in range(n, 0, -1):
            first = k // fact[i - 1]
            k %= fact[i - 1]
            ans += num[first]
            num.pop(first)
        return ans