[Leetcode][python]Permutation Sequence/第k个排列

class Solution(object):
    def getPermutation(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: str
        """
        curr = []
        self.result = ''
        for i in range(1, n+1):
            curr.append(i) 
        self.getPermutationHelper(n, k, curr)
        return self.result
    def getPermutationHelper(self, index, rest, curr):
        # print index, rest, '结果str：', curr
        if index == 0:
            return 
        for i in range(index-1, -1, -1):
            temp = math.factorial(index-1)
            # print temp, i, rest - (temp * i + 1)
            if rest - (temp * i + 1) >= 0:
                self.result += str(curr[i])
                curr.pop(i)
                # print(curr)
                self.getPermutationHelper(index-1, rest - (temp * i), curr)
                break

class Solution:

    def getPermutation(self, n, k):
        res = ''
        k -= 1
        fac = 1
        for i in range(1, n): fac *= i
        num = [1, 2, 3, 4, 5, 6, 7, 8, 9]
        for i in reversed(range(n)):
            curr = num[k/fac]
            res += str(curr)
            num.remove(curr)
            if i !=0:
                k %= fac
                fac /= i
        return res