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]
]

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