leetcode 102 Binary Tree Level Order Traversal ----- java

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]
]

 输出一棵树的每一行，很简单。

/**
 * 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 = new ArrayList<List<Integer>>();
        
        if( root == null)
            return list;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while( !queue.isEmpty() ){
            List ans = new ArrayList<Integer>();
            int size = queue.size();
            for( int i = 0;i<size;i++){
                TreeNode node = queue.poll();
                if( node == null)
                    continue;
                ans.add(node.val);
                queue.add(node.left);
                queue.add(node.right);
            }
            if( ans.size() != 0)
                list.add(ans);
            
        }
        
        
        return list;
    }
}