题解 | #求二叉树的层序遍历#

用一个helper方法进行DFS，每进去一层检查有没有这一层，没有就加多一层，然后将存在的点的值加到特定层中。

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     *
     * @param root TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */

    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        return levelOrderHelper(root,new ArrayList<ArrayList<Integer>>(),0);
    }
    public ArrayList<ArrayList<Integer>> levelOrderHelper(TreeNode root,ArrayList<ArrayList<Integer>> arr, int n){

        if(root != null){
            if(arr.size()<n+1){
                arr.add(new ArrayList<Integer>());
            }
            arr.get(n).add(root.val);
            levelOrderHelper(root.left,arr,n+1);
            levelOrderHelper(root.right,arr,n+1);
        }
        return arr;
    }
}