Problem102.java

import java.util.*;

class Problem102 {
	public static void main(String[] args) {
		TreeNode root = new TreeNode(1);
		root.left = new TreeNode(2);
		root.right = new TreeNode(3);

		System.out.println(new Problem102().levelOrder(root));
	}

	public List<List<Integer>> levelOrder(TreeNode root) {
 Queue<TreeNode> q = new LinkedList<>();
 List<List<Integer>> res = new ArrayList<>();
 if (root == null) return res;
 q.add(root);
 // 先取出队列中的数，并写入左右子节点
 while(q.size() > 0) {
 	List<Integer> lev = new ArrayList<>();
 	int size = q.size();
 	for (int i = 0; i < size; i++) {
 		TreeNode node = q.remove();
 		if (node.left != null) q.add(node.left);
 		if (node.right != null)q.add(node.right);
 		lev.add(node.val);
 	}
 	res.add(lev);
 }
 return res;
 }
}