All Nodes Distance K in Binary Tree

# 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 = defaultdict(list)
        
        def build_graph(node, parent):
            if node and parent:
                graph[node.val].append(parent.val)
                graph[parent.val].append(node.val)
            if node.left:
                build_graph(node.left, node)
            if node.right:
                build_graph(node.right, node)

        build_graph(root, None)

        visited = set([target.val])
        result = []
        queue = [(target.val, 0)]
        
        while queue:
            vertex, distance = queue.pop(0)
            
            if distance == k:
                result.append(vertex)
                continue
            
            for neighbor in graph[vertex]:
                if neighbor not in visited:
                    visited.add(neighbor)
                    queue.append((neighbor, distance + 1))
        
        return result