content: package lcof32_iii_cong_shang_dao_xia_da_yin_er_cha_shu;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Queue;
  
  public class Solution {
      public List<List<Integer>> levelOrder(TreeNode root) {
          List<List<Integer>> result = new ArrayList<>();
          if (null == root) {
              return result;
          }
          Queue<TreeNode> queue = new LinkedList<>();
          queue.add(root);
          boolean reverse = false;
          while (!queue.isEmpty()) {
              int qSize = queue.size();
              List<Integer> list = new ArrayList<>();
              for (int i = 0; i < qSize; i++) {
                  TreeNode now = queue.poll();
                  list.add(now.val);
                  if (null != now.left) {
                      queue.add(now.left);
                  }
                  if (null != now.right) {
                      queue.add(now.right);
                  }
              }
              if (reverse) {
                  Collections.reverse(list);
              }
              result.add(list);
              reverse = !reverse;
          }
          return result;
      }
  }