Leetcode 247: Strobogrammatic Number II

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

 

public class Solution {
    public IList<string> FindStrobogrammatic(int n) {
        return DFS(n, n);
    }
    
    private IList<string> DFS(int n, int m)
    {
        if (n == 0)
        {
            return new List<string>() {""};
        }
        
        if (n == 1)
        {
            return new List<string>() {"0", "1", "8"};
        }
        
        var list = DFS(n - 2, m);
        var result = new List<string>();
        
        foreach (var s in list)
        {
            if (n != m)
            {
                result.Add("0" + s + "0");
            }
            
            result.Add("1" + s + "1");
            result.Add("9" + s + "6");
            result.Add("8" + s + "8");
            result.Add("6" + s + "9");
        }
        
        return result;
    }
}

// backtracking, it didn't pass large test case
public class Solution1 {
    private int[] allCandidates = new int[] {0, 1, 6, 8, 9};
    private int[] centerCandidates = new int[] {0, 1, 8};
    private Dictionary<int, int> map = new Dictionary<int, int>();
 
    public IList<string> FindStrobogrammatic(int n) {
        map[0] = 0;
        map[1] = 1;
        map[6] = 9;
        map[8] = 8;
        map[9] = 6;
        
        var result = new List<string>();
        DFS(n, 0, new List<int>(), result);
        return result;
    }
    
    private void DFS(int n, int start, IList<int> res, IList<string> result)
    {
        if (start > n / 2)
        {
            var sb = new StringBuilder();
            
            for (int i = 0; i < res.Count; i++)
            {
                sb.Append(res[i]);
            }
            
            int s = n % 2 == 0 ? res.Count - 1 : res.Count - 2;
            for (int i = s; i >= 0; i--)
            {
                sb.Append(map[res[i]]);
            }
            
            result.Add(sb.ToString());
            
            return;
        }
            
        if (start == n / 2 && n % 2 != 0)
        {
            for (int i = 0; i < centerCandidates.Length; i++)
            {
                res.Add(centerCandidates[i]);
                DFS(n, start + 1, res, result);
                res.RemoveAt(res.Count - 1);
            }
        }
        else
        {
            for (int i = 0; i < allCandidates.Length; i++)
            {
                if (start == 0 && allCandidates[i] == 0) continue;
                
                res.Add(allCandidates[i]);
                DFS(n, start + 1, res, result);
                res.RemoveAt(res.Count - 1);
            }
        }
    }
}