Today I have done leetcode's February LeetCoding Challenge with cpp.

February LeetCoding Challenge 19

Description

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"

Constraints:

• 1 <= n <= 10
• 1 <= k <= 100

Solution

class Solution {
  void solve(string &result, int &count, int pos) {
    if(pos == result.length()) {
      count -= 1;
      return;
    }
    for(char c = 'a'; c < 'd' && count; ++c) {
      if(pos && result[pos - 1] == c) continue;
      result[pos] = c;
      solve(result, count, pos + 1);
    }
  }
public:
  string getHappyString(int n, int k) {
    string answer(n, 'a');
    solve(answer, k, 0);
    if(k) return "";
    return answer;
  }
};

// Accepted
// 345/345 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 87.05 % of cpp submissions (8 MB)