binary tree to undirected graph

treeToGraph

// binary tree to undirected graph
// treeToGraph(null, root);
public void treeToGraph(TreeNode parent, TreeNode child) {
    if (parent != null) {
        List<TreeNode> pNeighbors = g.getOrDefault(parent, new ArrayList<TreeNode>());
        pNeighbors.add(child);
        g.put(parent, pNeighbors);

        List<TreeNode> cNeighbors = g.getOrDefault(child, new ArrayList<TreeNode>());
        cNeighbors.add(parent);
        g.put(child, cNeighbors);
    }
    if (child.left != null) treeToGraph(child, child.left);
    if (child.right != null) treeToGraph(child, child.right);
}