1. 将从root到target的路线记录在deque中
2. 从后向前从deque中取出节点，每次从所取节点往下做深度为i的dfs
3. node_vis用来防止走重复的路线

/**
 * 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> ans;
    TreeNode* tar, *node_vis;
    deque<TreeNode*> q;

    bool dfs_findTar(TreeNode* node){
        if (node == nullptr)    return false;
        q.push_back(node);
        if (node == tar)
            return true;

        if (dfs_findTar(node->left)) return true;
        if (dfs_findTar(node->right)) return true;

        q.pop_back();
        return false;
    }

    void dfs_down(TreeNode* node, int h){
        if (node == nullptr)    return;
        if (h == 0) 
            ans.push_back(node->val);
        if (node->right != node_vis) dfs_down(node->right, h-1);
        if (node->left != node_vis) dfs_down(node->left, h-1);
    }

    vector<int> distanceK(TreeNode* root, TreeNode* target, int K) {
        tar = target;
        dfs_findTar(root);
        int cnt =-1;

        while (q.size()){
            cnt++;
            auto t = q.back(); q.pop_back();
            dfs_down(t, K - cnt);
            node_vis = t;
        }

        return ans;
    }
};