//time: O(n)

//space: o(n)

class Solution {

public List<List<Integer>> levelOrder(TreeNode root) {

List<List<Integer>> res = new ArrayList<>();

if(root == null) return res;

Queue<TreeNode> queue = new LinkedList<>();

queue.offer(root);

while(!queue.isEmpty()){

List<Integer> list = new ArrayList<>();

int size = queue.size();

for (int i = 0; i<size; i++){

TreeNode cur = queue.poll();

System.out.println(cur.val);

if(cur.left!=null) queue.offer(cur.left);

if(cur.right!= null) queue.offer(cur.right);

list.add(cur.val);

}

res.add(list);

}

return res;

}

}

class Solution {

public List<List<Integer>> levelOrder(TreeNode root) {

List<List<Integer>> res = new ArrayList<>();

if(root == null) return res;

helper(root, 0, res);

return res;

}

private void helper(TreeNode root, int level, List<List<Integer>>res){

if(root == null) return;

if(res.size() <= level){

res.add(new ArrayList<>());

}

res.get(level).add(root.val);

helper(root.left, level+1, res);

helper(root.right,level+1,res);

}

}