【牛客题霸每日一题】NC15 求二叉树的层序遍历
用dfs求解
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>> result = new ArrayList<>();
helper(root, result, 0);
return result;
}
private void helper(TreeNode root, ArrayList<ArrayList<Integer>> result, int depth) {
if (root == null) return;
if (result.size() == depth) result.add(new ArrayList<>());
result.get(depth).add(root.val);
helper(root.left, result, depth + 1);
helper(root.right, result, depth + 1);
}
} 
