面试题23：从上往下打印二叉树

importjava.util.ArrayList;

importjava.util.Deque;//注意引入包

importjava.util.LinkedList;

/**

public class TreeNode {

    int val = 0;

    TreeNode left = null;

    TreeNode right = null;

 

    public TreeNode(int val) {

        this.val = val;

 

    }

 

}

*/

publicclass Solution {

     publicArrayList<Integer> PrintFromTopToBottom(TreeNode root) {

         ArrayList<Integer> list = newArrayList<Integer>();

         if(root == null) returnlist;

         Deque<TreeNode> deque = newLinkedList<TreeNode>();

         

         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);

         }

         returnlist;

     }

}