Solution for Advent of Code Day 12 that (runs in ~150ms)

AdventOfCode_Day12.cs

var input = File.ReadAllText(Path.GetDirectoryName(Util.CurrentQueryPath) + @"\..\Inputs\aoc_day12.txt");
var lines = input.Split('\n').Select(line=>line.Trim()).ToArray();
var connections = new Dictionary<string,List<string>>();

foreach(var line in lines) 
{
	var nodeA = line.Split('-')[0];
	var nodeB = line.Split('-')[1];
	if(!connections.TryGetValue(nodeA, out var listA)) connections[nodeA] = listA = new List<string>();
	if(!connections.TryGetValue(nodeB, out var listB)) connections[nodeB] = listB = new List<string>();
	listA.Add(nodeB);
	listB.Add(nodeA);
}

int numPaths = 0;
var pathNodes = new List<string>();
bool hasUsedTwice = false;

DFS("start");
	
Console.WriteLine("count: " + numPaths);

void DFS(string node) 
{
	if(node == "end") 
	{
		numPaths++;
		return;
	}
	
	if(node == "start" && pathNodes.Count > 0) return; 
  
	bool isUpper = char.IsUpper(node[0]);
	bool storeUsedTwice = hasUsedTwice; 
	
	if(!isUpper && pathNodes.Contains(node))
	{
		if(hasUsedTwice) return;
		else hasUsedTwice = true; 
	}
	
	pathNodes.Add(node);
	foreach(var connection in connections[node]) DFS(connection);
	pathNodes.RemoveAt(pathNodes.Count-1);
	hasUsedTwice = storeUsedTwice;
}