Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<TreeNode>> answerList = new ArrayList<List<TreeNode>>();
        List<List<Integer>> list = new ArrayList<List<Integer>>();
        if (root == null) {
            return list;
        }
        List<TreeNode> initList = new ArrayList<TreeNode>();
        initList.add(root);
        answerList.add(initList);
        for (int i = 0; i < answerList.size(); i++) {
            List<Integer> l = new ArrayList<Integer>();
            List<TreeNode> newList = new ArrayList<TreeNode>();
            for (int j = 0; j < answerList.get(i).size(); j++) {
                TreeNode tree = answerList.get(i).get(j);
                l.add(answerList.get(i).get(j).val);
                if (tree.left != null) {
                    newList.add(tree.left);
                }
                if (tree.right != null) {
                    newList.add(tree.right);
                }
            }
            list.add(l);
            if (newList.size() != 0) {
                answerList.add(newList);
            }
        }
        return list;
    }
}