力扣-107题（Java）-层次遍历+Collections.reverse方法真香

Myth丶恋晨 2022-10-13 05:04 79阅读 0赞

题目链接：[https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/][https_leetcode-cn.com_problems_binary-tree-level-order-traversal-ii]  
题目如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwNDY3Njcw_size_16_color_FFFFFF_t_70]

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode() {}
     *     TreeNode(int val) { this.val = val; }
     *     TreeNode(int val, TreeNode left, TreeNode right) {
     *         this.val = val;
     *         this.left = left;
     *         this.right = right;
     *     }
     * }
     */
    class Solution {
        
        public List<List<Integer>> levelOrderBottom(TreeNode root) {
        
            List<List<Integer>> result=new ArrayList<List<Integer>>();
            if(root==null) return result;
    
            Queue<TreeNode> queue=new LinkedList<TreeNode>();
            queue.offer(root);
    
            while(!queue.isEmpty())
            {
        
                ArrayList<Integer> temp=new ArrayList<Integer>();
                int size=queue.size();
    
                while(size--!=0)
                {
        
                    TreeNode node=queue.poll();
    
                    temp.add(node.val);
                    if(node.left!=null) queue.offer(node.left);
                    if(node.right!=null) queue.offer(node.right);
                }
                result.add(temp);
            }
    
            Collections.reverse(result);//这个方法真香
    
            return result;
    
        }
    }

[https_leetcode-cn.com_problems_binary-tree-level-order-traversal-ii]: https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwNDY3Njcw_size_16_color_FFFFFF_t_70]: /images/20221005/e08589618e7642339ae21c0e33040571.png