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],

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

 题目含义：按照从左到右的顺序，输出每一层的节点

    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (!q.isEmpty())
        {
            int size = q.size();
            List<Integer> levels = new ArrayList<>();
            for (int i=0;i<size;i++)
            {
                TreeNode node = q.poll();
                levels.add(node.val);
                if (node.left !=null) q.offer(node.left);
                if (node.right !=null) q.offer(node.right);
            }
            result.add(levels);
        }
        return result;        
    }