刷题

class Solution {

public List<List<Integer>> levelOrderBottom(TreeNode root) {
    //解题思路：DFS遍历，注意结果采用头插法
    List<List<Integer>> result = new ArrayList();
    dfs(result, root, 0);
    return result;
}

private void dfs(List<List<Integer>> result, TreeNode root, int level){
    if(root == null){
        return;
    }
    //初始化新的层在第一行
    if(level == result.size()){
        result.add(0, new ArrayList());
    }
    //写入结果集;第level层要插入从后往前数的第level层
    result.get(result.size() - level - 1).add(root.val);
    
    //左子树
    dfs(result, root.left, level + 1);
    //右子树
    dfs(result, root.right, level + 1);
}

}