leetcode-60-第k个排列

题目描述：

方法一：

class Solution: 
    import math 
    def getPermutation(self, n: int, k: int) -> str: 
        result = '' 
        mod = k-1 
        if n==1:return '1' 
        n_set = [str(i) for i in range(1,n+1)] 
        fac_n = math.factorial(n-1) 
        for i in range(n): 
            val,mod = divmod(mod,fac_n) 
            #print(i,fac_n,val,mod,result,n_set) 
            fac_n = fac_n//(n-i-1) 
            result+=n_set[val] 
            del n_set[val] 
            if mod==0: 
                for i in n_set:
                    result+=i 
                return result 
        return result