102. Binary Tree Level Order Traversal

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,#,#,15,7},
[[3], [9,20], [15,7]]
tiyi:每层节点是一个子数组

代码：

public class Solution {    public List<List<Integer>> levelOrder(TreeNode root) {        List<List<Integer>> res = new ArrayList<List<Integer>>();        traversal(res, root, 0);        return res;    }    public void traversal(List<List<Integer>> res, TreeNode root, int height){        if(root == null) return;        if(height >= res.size()){            res.add(new LinkedList<Integer>());        }        res.get(height).add(root.val);        traversal(res, root.left, height+1);        traversal(res, root.right, height+1);    }}