4. (10%) 使用 queue 實作 level order traversal 轉換，運用以下 C 語言程式：
void level_order (tree *root) { //assume root is not null
node front, rear;
//實作以下敘述
// visit tree nodes based on level order by using queue
// use queue by front, rear, enqueue(&rear, root), root =dequeue(&front)
// print the values of tree nodes
// describe the ending condition
}