Problem URL: All nodes distance k in binary tree

First we will traverse the tree using DFS and create an adjacency list for each node. Then we will use BFS to find all nodes that are k distance away from the target node.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def distanceK(self, root: TreeNode, target: TreeNode, k: int) -> List[int]:
        graph = collections.defaultdict(list)
        def createGraph(parent, child):
            if parent and child:
                graph[parent.val].append(child.val)
                graph[child.val].append(parent.val)

            if child.left: 
                createGraph(child, child.left)
            if child.right: 
                createGraph(child, child.right)
        createGraph(None, root)

        visited = set([target.val])
        q = collections.deque([target.val])
        for _ in range(k):
            qLen = len(q)
            for i in range(qLen):
                node = q.pop()
                for nei in graph[node]:
                    if nei not in visited:
                        visited.add(nei)
                        q.appendleft(nei)

        return list(q)

Time complexity: O(n)
Space complexity: O(n)