Advent of Code - 2024 - Day 10

var input = await File.ReadAllLinesAsync("input.txt");

var gridSize = input.Length;
var nodes = new Node[gridSize, gridSize];
var trailHeads = new List<Node>();

for (var row = 0; row < gridSize; row++)
{
    for (var col = 0; col < gridSize; col++)
    {
        nodes[row, col] = new Node(input[row][col] - '0', []);
    }
}

for (var row = 0; row < gridSize; row++)
{
    for (var col = 0; col < gridSize; col++)
    {
        var node = nodes[row, col];
        var next = node.Value + 1;

        if (row > 0 && nodes[row - 1, col].Value == next)
        {
            node.Neighbors.Add(nodes[row - 1, col]);
        }

        if (col > 0 && nodes[row, col - 1].Value == next)
        {
            node.Neighbors.Add(nodes[row, col - 1]);
        }

        if (row < gridSize - 1 && nodes[row + 1, col].Value == next)
        {
            node.Neighbors.Add(nodes[row + 1, col]);
        }

        if (col < gridSize - 1 && nodes[row, col + 1].Value == next)
        {
            node.Neighbors.Add(nodes[row, col + 1]);
        }

        if (node.Value == 0)
        {
            trailHeads.Add(node);
        }
    }
}

Console.WriteLine($"Part 1: {WalkTrails(true)}");
Console.WriteLine($"Part 2: {WalkTrails(false)}");

return;

int WalkTrails(bool part1)
{
    var result = 0;
    foreach (var trailHead in trailHeads)
    {
        //DFS
        var stack = new Stack<Node>();
        stack.Push(trailHead);
        var visited = new HashSet<Node>();

        while (stack.Count > 0)
        {
            var current = stack.Pop();

            if (part1 && !visited.Add(current))
            {
                continue; //Part 1: We only count each node once.
            }

            if (current.Value == 9)
            {
                result++;
                continue;
            }

            foreach (var neighbor in current.Neighbors)
            {
                stack.Push(neighbor);
            }
        }
    }

    return result;
}

internal record Node(int Value, List<Node> Neighbors);