Serialization Pattern

Serialization converts a tree to a string; deserialization reconstructs it.

A common approach uses preorder with null markers:

function serialize(root):
    if root == null:
        return "null,"
    return toString(root.val) + "," + serialize(root.left) + serialize(root.right)

Result for tree $[1,2,3]$: "1,2,null,null,3,null,null,"

Deserialization processes the string left to right:

function deserialize(data):
 values = data.split(",")
 i = 0
 function build():
 val = values[i]
     i += 1
 if val == "null":
 return null
 node = TreeNode(toInt(val))
 node.left = build()
 node.right = build()
 return node
 return build()

The preorder sequence with null markers captures structure completely.