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

February LeetCoding Challenge 16

Description

Construct the Lexicographically Largest Valid Sequence

Given an integer n, find a sequence that satisfies all of the following:

• The integer 1 occurs once in the sequence.
• Each integer between 2 and n occurs twice in the sequence.
• For every integer i between 2 and n, the distance between the two occurrences of i is exactly i.

The distance between two numbers on the sequence, a[i] and a[j], is the absolute difference of their indices, |j - i|.

Return the lexicographically largest sequence. It is guaranteed that under the given constraints, there is always a solution.

A sequence a is lexicographically larger than a sequence b (of the same length) if in the first position where a and b differ, sequence a has a number greater than the corresponding number in b. For example, [0,1,9,0] is lexicographically larger than [0,1,5,6] because the first position they differ is at the third number, and 9 is greater than 5.

Example 1:

Input: n = 3
Output: [3,1,2,3,2]
Explanation: [2,3,2,1,3] is also a valid sequence, but [3,1,2,3,2] is the lexicographically largest valid sequence.

Example 2:

Input: n = 5
Output: [5,3,1,4,3,5,2,4,2]

Constraints:

• 1 <= n <= 20

Solution

class Solution {
  vector<bool> vis;
  vector<int> answer;
  int n;
  bool solve(int index) {
    if(index == 2 * n - 1) return true;
    // cout << "#" << answer << endl;
    if(answer[index]) return solve(index + 1);
    for(int i = min(n, 2 * n - 2 - index); i > 1 ; --i) {
      // cout << index << ' ' << i << endl;
      if(vis[i]) continue;
      // cout << "???" << endl;
      if(i != 1 && answer[i + index]) continue;
      // cout << "????" << endl;
      answer[index] = i;
      answer[index + i] = i;
      vis[i] = true;
      if(solve(index + 1)) return true;
      answer[index] = 0;
      answer[index + i] = 0;
      vis[i] = false;
    }
    if(!vis[1]) {
      answer[index] = 1;
      vis[1] = true;
      if(solve(index + 1)) return true;
      vis[1] = false;
      answer[index] = 0;
    }
    return false;
  }
public:
  vector<int> constructDistancedSequence(int n) {
    this->n = n;
    vis.resize(n + 1);
    answer.resize(n * 2 - 1);
    solve(0);
    return answer;
  }
};

// Accepted
// 20/20 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 36.8 % of cpp submissions (8.6 MB)