1using System;
2using System.Collections.Generic;
3
4public class TreeNode {
5    public int val;
6    public TreeNode left;
7    public TreeNode right;
8    public TreeNode(int x) { val = x; }
9}
10
11public class Solution {
12    public int AmountOfTime(TreeNode root, int start) {
        var graph = new Dictionary<int, List<int>>();
        BuildGraph(root, -1, graph);
        var visited = new HashSet<int>();
        var queue = new Queue<int>();
        queue.Enqueue(start);
        visited.Add(start);
        int time = 0;

        while (queue.Count > 0) {
            int size = queue.Count;
            for (int i = 0; i < size; i++) {
                int node = queue.Dequeue();
                foreach (var neighbor in graph[node]) {
                    if (!visited.Contains(neighbor)) {
                        visited.Add(neighbor);
                        queue.Enqueue(neighbor);
                    }
                }
            }
            if (queue.Count > 0) time++;
        }
        return time;
    }

    private void BuildGraph(TreeNode node, int parent, Dictionary<int, List<int>> graph) {
        if (node == null) return;
        if (parent != -1) {
            if (!graph.ContainsKey(node.val)) graph[node.val] = new List<int>();
            if (!graph.ContainsKey(parent)) graph[parent] = new List<int>();
            graph[node.val].Add(parent);
            graph[parent].Add(node.val);
        }
        BuildGraph(node.left, node.val, graph);
        BuildGraph(node.right, node.val, graph);
    }
}