import java.util.*; /* * public class TreeNode { * int val = 0; * TreeNode left = null; * TreeNode right = null; * } */ public class Solution { /** * * @param root TreeNode类 * @return int整型ArrayList<ArrayList<>> */ public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) { // write code here ArrayList<ArrayList<Integer>> treeList = new ArrayList<>(); if(root == null){ return treeList; } else { Queue<TreeNode> queue = new LinkedList<>(); queue.offer(root); while(!queue.isEmpty()){ ArrayList<Integer> list = new ArrayList<>(); int size = queue.size(); int count = 0; while(count<size){ TreeNode node = queue.poll(); list.add(node.val); // 出队一个元素 if(node.left!=null){ queue.offer(node.left); } if(node.right!=null){ queue.offer(node.right); } count++; } treeList.add(list); } } return treeList; } }