剑指Offer32.从上到下打印二叉树

题目链接：从上到下打印二叉树

思路：队列

代码：

/**
 * 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];
        List<Integer> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode q=queue.poll();
            res.add(q.val);
            if(q.left!=null) queue.offer(q.left);
            if(q.right!=null) queue.offer(q.right);
        }
        int[] t = new int[res.size()];
        for(int i=0; i<res.size(); i++){
            t[i] = res.get(i);
        }
        return t;
    }
}