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

Problem

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"

  Constraints:

• 1 <= n <= 10

• 1 <= k <= 100

Solution (Java)

class Solution {
    private char[] arr = new char[] {'a', 'b', 'c'};
    private String res = "";
    private int k;

    private void get(StringBuilder str, int n, int index) {
        if (k < 1) {
            return;
        }
        if (str.length() == n) {
            if (k == 1) {
                res = str.toString();
            }
            k--;
            return;
        }
        for (int i = 0; i < 3; i++) {
            if (i == index) {
                continue;
            }
            str.append(arr[i]);
            get(str, n, i);
            str.deleteCharAt(str.length() - 1);
        }
    }

    public String getHappyString(int n, int k) {
        this.k = k;
        get(new StringBuilder(), n, -1);
        return res;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).