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]]

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {       public List<List<Integer>> levelOrder(TreeNode root) {        List<List<Integer>> list = new ArrayList<List<Integer>>();        if (root == null)            return list;        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.add(root);        while (queue.size() != 0) {            List<Integer> alist = new ArrayList<Integer>();            for (TreeNode child : queue)                alist.add(child.val);            list.add(new ArrayList<Integer>(alist));            Queue<TreeNode> queue2=queue;            queue=new LinkedList<TreeNode>();            for(TreeNode child:queue2){                if (child.left != null)                    queue.add(child.left);                if (child.right != null)                    queue.add(child.right);            }        }        return list;    }}