用一个size记录每一层要弹出多少个， 我知道每一轮我要搞几次

    1.  public List<List<Integer>> levelOrder(TreeNode root) {
    2.      List<List<Integer>> ans = new ArrayList<>();
    3.      if (root == null) {
    4.          return ans;
    5.      }
    6.      Queue<TreeNode> queue = new LinkedList<>();
    8.      TreeNode node = root;
    9.      queue.offer(node);
    10.      int size = 0;
    11.      while (!queue.isEmpty()) {
    12.          size = queue.size();
    13.          List<Integer> curLevel = new ArrayList<>();
    14.          for (int i = 0; i < size; i++) {
    15.              node = queue.poll();
    16.              curLevel.add(node.val);
    17.              if (node.left != null) {
    18.                  queue.add(node.left);
    19.              }
    20.              if (node.right != null) {
    21.                  queue.add(node.right);
    22.              }
    23.          }
    24.          ans.add(curLevel);
    25.      }
    26.      return ans;
    27.  }
    32. public class TreeNode {
    33.     int val;
    34.     TreeNode left;
    35.     TreeNode right;
    36.     TreeNode() {}
    37.     TreeNode(int val) { this.val = val; }
    38.     TreeNode(int val, TreeNode left, TreeNode right) {
    39.         this.val = val;
    40.         this.left = left;
    41.         this.right = right;
    42.     }
    43. }