/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> ret = new ArrayList<List<Integer>>();
if (root == null) {
return ret;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
List<Integer> level = new ArrayList<Integer>();
int currentLevelSize = queue.size();
for (int i = 1; i <= currentLevelSize; ++i) {
TreeNode node = queue.poll();
level.add(node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
ret.add(level);
}
return ret;
}
}