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
Solution:
class Solution {
public String getHappyString(int n, int k) {
Deque<String> queue = new ArrayDeque();
queue.offer("");
int count = 0;
while (!queue.isEmpty()) {
String curr = queue.poll();
if (curr.length() == n) {
count ++;
if (count == k) return curr;
}
if (curr.length() > n) {
return "";
}
char last = curr.length() > 0 ? curr.charAt(curr.length() - 1) : '#';
if (last != 'a') {
queue.offer(curr + "a");
}
if (last != 'b') {
queue.offer(curr + "b");
}
if (last != 'c') {
queue.offer(curr + "c");
}
}
return "";
}
}