hardDSA

How do you serialize and deserialize a binary tree?

Q: How do you serialize and deserialize a binary tree?

**Intuition.** Turn the tree into a string and back without losing structure. A **preorder traversal** that emits a sentinel (like # ) for every null child fully captures the shape — because nulls mark exactly where subtrees end. Deserialization replays that preorder, consuming tokens in the same order. preorder with null markers fully encodes the shape

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Explain tree serialization.

treesserialization

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Preorder serialize/deserialize

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

def serialize(root):
    out = []
    def dfs(node):
        if not node:
            out.append('#')
            return
        out.append(str(node.val))
        dfs(node.left)
        dfs(node.right)
    dfs(root)
    return ','.join(out)

def deserialize(data):
    tokens = iter(data.split(','))
    def build():
        val = next(tokens)
        if val == '#':
            return None
        node = TreeNode(int(val))
        node.left = build()
        node.right = build()
        return node
    return build()


# --- demo ---
root = TreeNode(1, TreeNode(2), TreeNode(3, TreeNode(4), TreeNode(5)))
data = serialize(root)
print(data)                          # 1,2,#,#,3,4,#,#,5,#,#
print(serialize(deserialize(data)))  # same string -> round-trip ok

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