题解 | #求二叉树的层序遍历#

通过二叉树遍历添加节点，不必刻意一层层遍历，维护一个floor层数即可
需要注意的地方：l.size()其实就代表着层数，在floor大于l.size()的时候说明进入了新的层，创建一个ArrayList<integer>添加到l中</integer>

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
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        preOrder(root, res, 1);
        return res;
    }

    private void preOrder(TreeNode root, ArrayList<ArrayList<Integer>> l, int floor) {
        if (root == null) {
            return;
        }
        if (floor > l.size()) {
            ArrayList<Integer> addList = new ArrayList<>();
            l.add(addList);
        }
        l.get(floor - 1).add(root.val);
        preOrder(root.left, l, floor + 1);
        preOrder(root.right, l, floor + 1);
    }
}