leetcode_102

题目来源：https://leetcode.com/problems/binary-tree-level-order-traversal/

这道题和107题刚好是相反的，所以将每层节点的值加入最终结果的Arraylist的尾部即可

代码：

/**

• Definition for a binary tree node.
• public class TreeNode {
• int val;
• TreeNode left;
• TreeNode right;
• TreeNode(int x) { val = x; }
• }
  */
  class Solution {
  public List<List> levelOrder(TreeNode root) {

  Queue<TreeNode> queue = new LinkedList<>();
  List<List<Integer>> result = new ArrayList<>();
  queue.add(root);
  while(!queue.isEmpty()){
      List<Integer> currentResult = new ArrayList<>();
      int size = queue.size();
      for(int i = 0;i < size;i++){
          TreeNode node = queue.remove();
          if (node != null){
              currentResult.add(node.val);
              queue.add(node.left);
              queue.add(node.right);
          }
      }
      if(!currentResult.isEmpty()){
          result.add(currentResult);
      }
  }
  return result;
  

  }
  }