Lexicographic permutations
Problem 24 A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are:
012 021 102 120 201 210
What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9?
思路：从最高位开始，最高位0，1，2，3，每个选择都有10!/10种可能，那就1000000/(factorial(10)/10)可以确定选第几个数字，迭代获取每一位的数字，要注意我们这样获取的是第999999位数字，所以，除数要+1
代码：swift
var c:Double = 1000000+1
var arr:Array = [0,1,2,3,4,5,6,7,8,9]
var dic:[Int:Int] = [:]
for i in 0...9 {
let jiechen = self.jiechen(10-i)/Double(10-i)
let index = Int(c/jiechen)
dic[9-i] = arr[index]
arr.remove(at: index)
let ddd = c.truncatingRemainder(dividingBy: jiechen)
print(i)
c = ddd
}
print(dic)
var numArray:Array<Int>=[]
for i in 0...9 {
numArray.insert(dic[i]!, at: 0)
}
print(numArray)
