//---------------------------------------------------------------------
//------------- B F S + I T E R A T I V E -------------------
//---------------------------------------------------------------------
//BFS: Level-order Traversal, Queue
public List<List<int>> LevelOrderTraversal(TreeNode root)
{
List<List<int>> result = new List<List<int>>();
Queue<TreeNode> queue = new Queue<TreeNode>();
queue.Enqueue(root);
while (queue.Count != 0)
{
//Initiate a List<int> container for each level.
List<int> level = new List<int>();
//Store the static size as loop condition (this is a must because queue.Count is shrinking in each loop.)
int size = queue.Count;
for (int i = 0; i < size; i++)
{
//Dequeue each node
root = queue.Dequeue();
//Save the node value
level.Add(root.val);
//And enqueue its child at the same time
if (root.left != null) queue.Enqueue(root.left);
if (root.right != null) queue.Enqueue(root.right);
}
//Save the level after dequeue all the node.
result.Add(level);
}
return result;
}