利用队列

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

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