利用Queue——LinkedList
    queue.offer()
    quque.poll()
    queue.peek()

    1. import java.util.*;
    3. /*
    4.  * public class TreeNode {
    5.  *   int val = 0;
    6.  *   TreeNode left = null;
    7.  *   TreeNode right = null;
    8.  * }
    9.  */
    11. public class Solution {
    12.     /**
    13.      * 
    14.      * @param root TreeNode类 
    15.      * @return int整型ArrayList<ArrayList<>>
    16.      */
    17.     public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
    18.         // write code here
    19.         ArrayList<ArrayList<Integer>> res= new ArrayList<>();
    20.         if(root == null){
    21.             return res;
    22.         }
    24.         Queue<TreeNode> queue = new LinkedList<TreeNode>();
    25.         queue.offer(root);
    27.         while(!queue.isEmpty()){
    28.             ArrayList<Integer> level = new ArrayList<Integer>();
    29.             int levelNum = queue.size();
    30.             for(int i=0; i<levelNum; i++){
    31.                 if(queue.peek().left!=null){
    32.                     queue.offer(queue.peek().left);
    33.                 }
    34.                 if(queue.peek().right!=null){
    35.                     queue.offer(queue.peek().right);
    36.                 }
    37.                 level.add(queue.poll().val);
    38.             }
    39.             res.add(level);
    40.         }
    41.         return res;
    43.     }
    44. }