劍指之從上往下打印二叉樹

從上往下打印出二叉樹的每個節點，同層節點從左至右打印

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
/**
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> list = new ArrayList<Integer>();
         Deque<TreeNode> deque = new LinkedList<TreeNode>();
         if(root == null) 
              return list;
         deque.add(root);
         while(!deque.isEmpty()){
             TreeNode t = deque.pop();
             list.add(t.val);
             if(t.left != null) 
                    deque.add(t.left);
             if(t.right != null) 
                   deque.add(t.right);
         }
         return list;
    }
}