题解 | #求二叉树的层序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
        level(list,0,root);
        return list;
    }

    private void level(ArrayList<ArrayList<Integer>> list, int i, TreeNode root) {
        // TODO
        if(root==null){
            return;
        }
        if(list.size()<=i){
            list.add(new ArrayList<Integer>());
        }
        list.get(i).add(root.val);
        level(list,i+1,root.left);
        level(list,i+1,root.right);
    }
}