LeetCode in Kotlin

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

Medium

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 9^th string = “cab”

Constraints:

• 1 <= n <= 10
• 1 <= k <= 100

Solution

class Solution {
    private val arr = charArrayOf('a', 'b', 'c')
    private var res = ""
    private var k = 0

    private operator fun get(str: StringBuilder, n: Int, index: Int) {
        if (k < 1) {
            return
        }
        if (str.length == n) {
            if (k == 1) {
                res = str.toString()
            }
            k--
            return
        }
        for (i in 0..2) {
            if (i == index) {
                continue
            }
            str.append(arr[i])
            get(str, n, i)
            str.deleteCharAt(str.length - 1)
        }
    }

    fun getHappyString(n: Int, k: Int): String {
        this.k = k
        get(StringBuilder(), n, -1)
        return res
    }
}

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