leetcode之Permutation Sequence

这题的主要考点在于阶乘，排列是有序的之后，可以按照阶乘来逐步的求出每个数字。代码如下：

class Solution(object):
    def getPermutation(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: str
        """
        from math import factorial
        a = []
        for i in range(n):
            a.append(str(i + 1))
        b = []
        while k > 0:
            changer = k / factorial(n - 1)
            k = k % factorial(n - 1)
            if k == 0:
                b.append(a[changer - 1])
                del a[changer - 1]
                return ''.join(b) + ''.join(a)[::-1]
            else:
                b.append(a[changer])
                del a[changer]
                n = n - 1