102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

一刷
题解：DFS，并在子函数内传入level表示加入哪一层list

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null) return res;
        level(root, 0, res);
        return res;
    }
    
    public void level(TreeNode root, int level, List<List<Integer>> res){
        if(root == null) return;
        if(level>=res.size()) res.add(new ArrayList<>());
        res.get(level).add(root.val);
        level(root.left, level+1, res);
        level(root.right, level+1, res);
    }
}