863. All Nodes Distance K in Binary Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> distanceK(TreeNode* root, TreeNode* target, int K) {
        vector<bool> visited(501, false);
        vector<unordered_set<int>> g(501);
        
        dfs(root, g);
        
        vector<int> res;
        queue<int> q;
        q.push(target->val);
        visited[target->val] = true;
        int lv = 0;
        while (!q.empty()) {
            int qs = q.size();
            if (lv == K) {
                while(!q.empty()) {
                    res.push_back(q.front());
                    q.pop();
                }
                break;
            }
            while (qs-- > 0) {
                int p = q.front();
                q.pop();
                for (auto i : g[p]) {
                    if (!visited[i]) {
                        q.push(i);
                        visited[i] = true;
                    }
                }
            }
            lv++;
        }
        return res;
    }
    void dfs(TreeNode *root, vector<unordered_set<int>>& g) {
        if (root == NULL)   return;
        if (root->left) {
            g[root->val].insert(root->left->val);
            g[root->left->val].insert(root->val);
        }
        if (root->right) {
            g[root->val].insert(root->right->val);
            g[root->right->val].insert(root->val);
        }
        dfs(root->left, g);
        dfs(root->right, g);
    }
};