二叉树的层次遍历

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