440. K-th Smallest in Lexicographical Order

Given two integers n and k, return the kth lexicographically smallest integer in the range [1, n].

Example 1:

Input: n = 13, k = 2
Output: 10
Explanation: The lexicographical order is [1, 10, 11, 12, 13, 2, 3, 4, 5, 6, 7, 8, 9], so the second smallest number is 10.

Example 2:

Input: n = 1, k = 1
Output: 1

def findKthNumber(self, n: int, k: int) -> int:
	return self.solve(0, n, k)

def solve(self, current, n, k):
	if k == 0:
		return current // 10

	for i in range(max(current, 1), current + 10):
		count = self.numOfChildren(i, i + 1, n)
		if count >= k:
			return self.solve(i * 10, n, k - 1)
		k -= count

	return -1  

def numOfChildren(self, current, neighbour, n):
	if neighbour > n:
		if current > n:
			return 0
		return n - current + 1
	return (neighbour - current) + self.numOfChildren(current * 10, neighbour * 10, n)