serialization on Binary Tree

Question

Implement a function to serialize a binary tree into a string representation and a function to deserialize a string back into the original binary tree structure. The serialization should produce a format that allows for easy reconstruction of the tree, and the input/output should handle edge cases such as empty trees.

Codemia · Accepted Answer

To serialize and deserialize a binary tree, we need to traverse the tree and represent its structure in a way that allows us to reconstruct it later. A pre-order traversal (root-left-right) is appropriate for serialization because it captures the root node before its children, ensuring we can reconstruct the tree correctly. The use of a delimiter (like a comma) allows us to distinguish between node values, while a placeholder (like 'null') indicates the absence of a child node.

1. **Serialization**: We will use a pre-order traversal. For each node, we append its value to a list. If a node is null, we append 'null'. At the end, we join the list into a single string with commas as separators.

**Example**: Given the tree:
   
       1
      / \
     2   3
    / \
   4   null
   
   - The pre-order traversal would be: '1,2,4,null,null,3,null,null'.
   
2. **Deserialization**: We will split the serialized string by commas and use an index to create the tree recursively. For each value, if it's 'null', we return null; otherwise, we create a new node and recursively construct its left and right children.
   
   **Example**: For the string '1,2,4,null,null,3,null,null', we would create nodes as we parse the string, resulting in the original tree structure.

```python
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

class Codec:
    def serialize(self, root):
        def helper(node):
            if node is None:
                return 'null'
            return str(node.val) + ',' + helper(node.left) + ',' + helper(node.right)
        return helper(root)

def deserialize(self, data):
        def helper(values):
            value = next(values)
            if value == 'null':
                return None
            node = TreeNode(int(value))
            node.left = helper(values)
            node.right = helper(values)
            return node
        return helper(iter(data.split(',')))
```

The time complexity for both serialization and deserialization is O(n), where n is the number of nodes in the tree. This is because we visit each node exactly once during the traversal. The space complexity is also O(n) due to the space needed to store the serialized string and the recursion stack during deserialization. If we consider the input size as the size of the serialized string, the space used will directly correspond to the number of nodes.

1. **Empty Tree**: For an empty tree (root is None), the serialization function should return 'null', and the deserialization function should return None when given 'null'.
   
2. **Single Node**: For a tree with a single node, the serialization should produce a string with just that node's value, e.g., '1', and deserialization should correctly reconstruct the single node tree.
   
3. **All Nodes to the Left/Right**: Trees that are skewed (all left or all right children) should still be handled correctly, producing a serialization that reflects the structure accurately.

serialization on Binary Tree

Twitter/X

What the Interviewer Expects

Key Topics to Cover

How to Approach This

Possible Follow-up Questions

Sharpen Your Skills on Codemia

Sample Answer

Problem Analysis

Approach

Submit Your Answer

Twitter/X Software Engineer Interview Guide

Related Questions

Binary Search on graph

Count kth largest element in array

Find connected components in string

Merge Three Sorted strings

Binary Search on string