二叉树的层次遍历

public class Solution {
public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
ArrayList result = new ArrayList();

if (root == null) {
return result;
}

Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);

while (!queue.isEmpty()) {
ArrayList<Integer> level = new ArrayList<Integer>();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode head = queue.poll();
level.add(head.val);
if (head.left != null) {
queue.offer(head.left);
}
if (head.right != null) {
queue.offer(head.right);
}
}
result.add(level);
}

return result;
}
}