README.md

1415. The k-th Lexicographical String of All Happy Strings of Length n

A happy string is a string that:

• consists only of letters of the set ['a', 'b', 'c'].
• s[i] != s[i + 1] for all values of i from 1 to s.length - 1 (string is 1-indexed).

For example, strings "abc", "ac", "b" and "abcbabcbcb" are all happy strings and strings "aa", "baa" and "ababbc" are not happy strings.

Given two integers n and k, consider a list of all happy strings of length n sorted in lexicographical order.

Return the kth string of this list or return an empty string if there are less than k happy strings of length n.

Example 1:

Input: n = 1, k = 3
Output: "c"
Explanation: The list ["a", "b", "c"] contains all happy strings of length 1. The third string is "c".

Example 2:

Input: n = 1, k = 4
Output: ""
Explanation: There are only 3 happy strings of length 1.

Example 3:

Input: n = 3, k = 9
Output: "cab"
Explanation: There are 12 different happy string of length 3 ["aba", "abc", "aca", "acb", "bab", "bac", "bca", "bcb", "cab", "cac", "cba", "cbc"]. You will find the 9th string = "cab"

Example 4:

Input: n = 2, k = 7
Output: ""

Example 5:

Input: n = 10, k = 100
Output: "abacbabacb"

Constraints:

• 1 <= n <= 10
• 1 <= k <= 100

Solutions (Python)

1. Solution

class Solution:
    def getHappyString(self, n: int, k: int) -> str:
        happy = list("abc")

        for _ in range(n - 1):
            happy_ = []

            for s in happy:
                for c in "abc":
                    if s[-1] != c:
                        happy_.append(s + c)

            happy = happy_

        return "" if k > len(happy) else happy[k - 1]

Solutions (Ruby)

1. Solution

# @param {Integer} n
# @param {Integer} k
# @return {String}
def get_happy_string(n, k)
  happy = %w[a b c]

  (2..n).each do |_i|
    happy_ = []

    happy.each do |s|
      'abc'.chars.each do |c|
        happy_.push(s + c) if s[-1] != c
      end
    end

    happy = happy_
  end

  k > happy.size ? '' : happy[k - 1]
end