README.md

Idea

For a tree travesal problem, we have two approaches to takle it: BFS and DFS.

For a BFS, we just use a queue, it is the same as 102. Binary Tree Level Order Traversal, the code is following:

res = []
queue = deque([root])
while queue:
		node = queue.popleft()
		if node:
				res.append(node.val)
				queue.extend([node.left, node.right])
return res

By changing the queue to stack, it will naturally give us DFS search. This corresponds to reorder:

res = []
stack = [root]
while stack:
		node = stack.pop()
		if node:
				stack.extend([node.right, node.left])
				res.append(node.val)
return res