1 import java.util.ArrayList; 2 import java.util.LinkedList; 3 import java.util.Queue; 4 /** 5 public class TreeNode { 6     int val = 0; 7     TreeNode left = null; 8     TreeNode right = null; 9 10     public TreeNode(int val) {11         this.val = val;12 13     }14 15 }16 */17 public class Solution {18     public ArrayList
     
       PrintFromTopToBottom(TreeNode root) {19 20         ArrayList
      
        list = new ArrayList
       
        ();21         22         Queue
        
          queue = new LinkedList
         
          ();23         24         queue.offer(root); //根结点入queue25         26         while(root != null){27             TreeNode node = queue.poll(); //结点出queue28             list.add(node.val);29             30             //结点的左右孩子入queue31             if(root.left != null){32                 queue.offer(root.left);33             }34             if(root.right != null){35                 queue.offer(root.right);36             }37             38             root = queue.peek();39         }40 41         return list;42     43     }44 }