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]
]

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null)
            return ans;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (q.size() != 0) {
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            while (size-- > 0) {
                TreeNode node = q.poll();
                level.add(node.val);
                if (node.left != null)
                    q.add(node.left);
                if (node.right != null)
                    q.add(node.right);
            }
            ans.add(level);
        }
        return ans;
    }
}