Leetcode102. 二叉树的层序遍历

题目传送：​​https://leetcode.cn/problems/binary-tree-level-order-traversal/submissions/​​

运行效率

代码如下：

class Solution {
     public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    // 处理边界情况
    if (root == null) {
      return result;
    }
    //层序遍历问题一般都要结合queue来解决
    ArrayDeque<TreeNode> queue = new ArrayDeque<>();
    queue.add(root);
    while (queue.size() > 0) {
      // arrayDeque用于存放下一层层的所有节点
      Deque<TreeNode> arrayDeque = new ArrayDeque<>(queue.size());
      // arrayList存放本层的所有节点值
      List<Integer> arrayList = new ArrayList<>();
      while (queue.size() > 0) {
        TreeNode poll = queue.poll();
        if(poll.left!=null){
          arrayDeque.add(poll.left);
        }
        if(poll.right!=null){
          arrayDeque.add(poll.right);
        }
        arrayList.add(poll.val);
      }
      result.add(arrayList);
     queue.addAll(arrayDeque);
    }
    return  result;
  }