Leetcode 754. Cracking the Safe

There is a box protected by a password. The password is n digits, where each letter can be one of the first k digits 0, 1, ..., k-1.

You can keep inputting the password, the password will automatically be matched against the last n digits entered.

For example, assuming the password is "345", I can open it when I type "012345", but I enter a total of 6 digits.

Please return any string of minimum length that is guaranteed to open the box after the entire string is inputted.

Example 1:

Input: n = 1, k = 2
Output: "01"
Note: "10" will be accepted too.

 

Example 2:

Input: n = 2, k = 2
Output: "00110"
Note: "01100", "10011", "11001" will be accepted too.

 

Note:

1. n will be in the range [1, 4].
2. k will be in the range [1, 10].
3. k^n will be at most 4096.

public class Solution {
    public string CrackSafe(int n, int k) {
        var sb = new StringBuilder();
        
        for (int i = 0; i < n; i++)
        {
            sb.Append('0');
        }
        
        var visited = new HashSet<string>();
        visited.Add(sb.ToString());
        
        DFS(n, k, visited, (int)Math.Pow(k, n), sb);
        
        return sb.ToString();
    }
    
    private bool DFS(int n, int k, HashSet<string> visited, int total, StringBuilder sb)
    {
        if (visited.Count >= total)
        {
            return true;
        }
        
        var prev = sb.ToString(sb.Length - n + 1, n - 1);
        
        for (int i = 0; i < k; i++)
        {
            var cur = prev + i.ToString();
            if (!visited.Contains(cur))
            {
                visited.Add(cur);
                
                sb.Append(i);
                
                if (DFS(n, k, visited, total, sb))
                {
                    return true;
                }
                else
                {
                    visited.Remove(cur);
                    sb.Remove(sb.Length - 1, 1);
                }
            }
        }
        
        return false;
    }
}