剑指offer(17)层次遍历树

public class Solution {
    
    ArrayList<Integer> list = new ArrayList<Integer>();
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        if(root==null)
            return list;
        TreeNode node = root;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(node);
        while(!q.isEmpty()){
            node = q.poll();
            if(node.left!=null)
                q.offer(node.left);
            if(node.right!=null)
                q.offer(node.right);
            list.add(node.val);
        }
        return list;
    }
}