剑指 Offer 32 - I. 从上到下打印二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int[] levelOrder(TreeNode root) {
        if(root == null) return new int[0];
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<Integer> list = new ArrayList<>();
        queue.offer(root);

        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            list.add(node.val);
            if(node.left != null) queue.offer(node.left);
            if(node.right != null) queue.offer(node.right);
        }

        int[] res = new int[list.size()];
        for(int i = 0; i < list.size(); i++)
            res[i] = list.get(i);
        return res;
    }
}