剑指 Offer 32 - I. 从上到下打印二叉树

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public static int[] levelOrder(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if (root == null) {
            return new int[]{};
        }
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()){
            TreeNode node=queue.poll();
            list.add(node.val);
            if(node.left!=null){
                queue.add(node.left);
            }
            if(node.right!=null){
                queue.add(node.right);
            }
        }
      	//list.stream().mapToInt(Integer::intValue).toArray();
        int[] res = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            res[i] =  list.get(i);
        }
        return res;
    }
}