serialize-and-deserialize-binary-tree.py

# Time:  O(n)
# Space: O(h)

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Codec(object):

    def serialize(self, root):
        """Encodes a tree to a single string.

        :type root: TreeNode
        :rtype: str
        """
        def serializeHelper(node):
            if not node:
                vals.append('#')
                return
            vals.append(str(node.val))
            serializeHelper(node.left)
            serializeHelper(node.right)
        vals = []
        serializeHelper(root)
        return ' '.join(vals)


    def deserialize(self, data):
        """Decodes your encoded data to tree.

        :type data: str
        :rtype: TreeNode
        """
        def deserializeHelper():
            val = next(vals)
            if val == '#':
                return None
            node = TreeNode(int(val))
            node.left = deserializeHelper()
            node.right = deserializeHelper()
            return node
        def isplit(source, sep):
            sepsize = len(sep)
            start = 0
            while True:
                idx = source.find(sep, start)
                if idx == -1:
                    yield source[start:]
                    return
                yield source[start:idx]
                start = idx + sepsize
        vals = iter(isplit(data, ' '))
        return deserializeHelper()


# time: O(n)
# space: O(n)

class Codec2(object):

    def serialize(self, root):
        """Encodes a tree to a single string.
        
        :type root: TreeNode
        :rtype: str
        """
        def gen_preorder(node):
            if not node:
                yield '#'
            else:
                yield str(node.val)
                for n in gen_preorder(node.left):
                    yield n
                for n in gen_preorder(node.right):
                    yield n
                
        return ' '.join(gen_preorder(root))
        
    def deserialize(self, data):
        """Decodes your encoded data to tree.
        
        :type data: str
        :rtype: TreeNode
        """
        def builder(chunk_iter):
            val = next(chunk_iter)
            if val == '#':
                return None
            node = TreeNode(int(val))
            node.left = builder(chunk_iter)
            node.right = builder(chunk_iter)
            return node
        
        # https://stackoverflow.com/a/42373311/568901
        chunk_iter = iter(data.split())
        return builder(chunk_iter)