Leetcode107. 二叉树的层序遍历 II

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

运行效率

代码如下

class Solution {
  public List<List<Integer>> levelOrderBottom(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
     //处理边界情况
    if(root==null){
      return result;
    }
    //层序遍历一般通过队列来实现
    ArrayDeque<TreeNode> deque = new ArrayDeque<>();
    deque.add(root);
    while (!deque.isEmpty()) {
      ArrayList<Integer> list = new ArrayList<>();
      // 用于存储下一层的节点
      ArrayDeque<TreeNode> arrayDeque = new ArrayDeque<>();
      while (!deque.isEmpty()) {
        TreeNode poll = deque.poll();
        list.add(poll.val);
        if (poll.left != null) {
          arrayDeque.add(poll.left);
        }
        if (poll.right != null) {
          arrayDeque.add(poll.right);
        }
      }
      result.add(list);
      deque.addAll(arrayDeque);
    }
    result=reverse(result);
    return  result;
  }
  //翻转顺序
  public List<List<Integer>> reverse(List<List<Integer>> list) {
    List<List<Integer>> result = new ArrayList<>();
    for (int i = list.size() - 1; i >= 0; i--) {
      result.add(list.get(i));
    }
    return  result;
  }
}