863. All Nodes Distance K in Binary Tree

We are given a binary tree (with root node root), a target node, and an integer value K.

Return a list of the values of all nodes that have a distance K from the target node. The answer can be returned in any order.

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], target = 5, K = 2

Output: [7,4,1]

Explanation:
The nodes that are a distance 2 from the target node (with value 5)
have values 7, 4, and 1.

Note that the inputs “root” and “target” are actually TreeNodes.
The descriptions of the inputs above are just serializations of these objects.

Note:

The given tree is non-empty.
Each node in the tree has unique values 0 <= node.val <= 500.
The target node is a node in the tree.
0 <= K <= 1000.

Code:

Tree Traversal for this problem is using Pre-order traversal. Once we find an target, we need to find all the kids related to this target node and save it into a pre list.

For find kids function’s conditions:

1. if the node is empty or if the node is visited, return pre list.
2. if the node hasn’t visited, set the visited[val] to true.
3. if the node has left sub-node, go check it and return updated pre.
4. if the node has right sub-node, go check it and return udpated pre.
5. once all of them are done, return pre.

For find alls finction’s condition:

1. find target node itself and return target node.
2. if nodeLogs has current node history, pull out the node and check find all kids for this node.
3. return pre list.

public class Solution {
    TreeNode[] nodeLogs;
    bool[] visited;
    public IList<int> DistanceK(TreeNode root, TreeNode target, int K) {
        IList<int> ans = new List<int>();
        nodeLogs = new TreeNode[501];
        visited = new bool[501];
        if(root == target) 
            return findKids(root, K, ans);
        Queue<TreeNode> queue = new Queue<TreeNode>();
        queue.Enqueue(root);
        while(queue.Any())
        {
            int size = queue.Count;
            for(int i = 0; i < size; i++)
            {
                TreeNode current = queue.Dequeue();
                if(current == target) 
                    return findAlls(current, K, ans);
                if(current.left != null)
                {
                    nodeLogs[current.left.val] = current;
                    queue.Enqueue(current.left);
                }
                if(current.right != null)
                {
                    nodeLogs[current.right.val] = current;
                    queue.Enqueue(current.right);
                }
            }
        }
        return ans;
    }
    private IList<int> findAlls(TreeNode target, int dis, IList<int> pre)
    {
        pre = findKids(target, dis, pre);
        if(nodeLogs[target.val] != null)
        {
            TreeNode pn = nodeLogs[target.val];
            pre = findAlls(pn, dis - 1, pre);
        }
        return pre;
    }
    private IList<int> findKids(TreeNode root, int dis, IList<int> pre)
    {
        if(root == null || visited[root.val]) return pre;
        visited[root.val] = true;
        if(dis == 0)
        {
            pre.Add(root.val);
            return pre;
        }
        pre = findKids(root.left, dis - 1, pre);
        pre = findKids(root.right, dis - 1, pre);
        return pre;
    }
}