# LeetCode 102. Binary Tree Level Order Traversal

Problem:

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]
]```

Solution:

BFS

```/**
* 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<Integer>> result = new ArrayList<List<Integer>>();
if (root == null) return result;
Queue<TreeNode> q = new LinkedList<TreeNode>();
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
List<Integer> row = new ArrayList<Integer>();
while (size > 0) {
TreeNode current = q.poll();
row.add(current.val);
if (current.left != null) q.offer(current.left);
if (current.right != null) q.offer(current.right);
size--;
}
result.add(row);
}
return result;
}
}
```