BFS
import java.util.*;
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
ArrayList<ArrayList<Integer> > Print(TreeNode root) {
ArrayList<ArrayList<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
ArrayList<Integer> list = new ArrayList<>();
while (size-- > 0) {
TreeNode cur = queue.poll();
list.add(cur.val);
if (cur.left != null) queue.offer(cur.left);
if (cur.right != null) queue.offer(cur.right);
}
res.add(list);
}
return res;
}
}