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Level order (Java)
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> levels = new ArrayList<List<Integer>>();
if (root == null) return levels;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
int level = 0;
while ( !queue.isEmpty() ) {
// start the current level
levels.add(new ArrayList<Integer>());
// number of elements in the current level
int level_length = queue.size();
for(int i = 0; i < level_length; ++i) {
TreeNode node = queue.remove();
// fulfill the current level
// add child nodes of the current level
// in the queue for the next level
if (node.left != null) queue.add(node.left);
if (node.right != null) queue.add(node.right);
// go to next level
return levels;
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