小白笔记----------------------------------------------leetcode(107. Binary Tree Level Order Traversal I)

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]
这道题主要就是二叉树的遍历，主要就是每一层都要标记一下，最后取一个反序。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List> levelOrderBottom(TreeNode root) {
        List> arr = new ArrayList();
        int level = 0;
        showTree(root,arr,0);
        List> arr1 = new ArrayList();
        for(int i = 0;i < arr.size();i++){
            arr1.add(arr.get(arr.size()-1-i));
        }
        return arr1;
    }
    public void showTree(TreeNode root, List> arr,int level){
        if(root == null){
            return; 
        }
        if(arr.size()-1 < level)
        {
            arr.add(new ArrayList());
        }
        arr.get(level).add(root.val);
        showTree(root.left,arr,level+1);
        showTree(root.right,arr,level+1);
           
        
       
    }
   
}