24. Serialize and Deserialize Binary Tree

hardAsked at Byju's

Convert a binary tree to a string and reconstruct the identical tree from that string.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Design an algorithm to serialize and deserialize a binary tree. Serialization is the process of converting a tree to a string so it can be stored or transmitted, and reconstructed identically later. You may pick any format as long as encode/decode are inverses.

Constraints

0 <= nodes <= 10^4
-1000 <= Node.val <= 1000

Examples

Example 1

Input

root = [1,2,3,null,null,4,5]

Output

tree reconstructed identically

Example 2

Input

root = []

Output

empty tree

Approaches

1. Level-order with index map

BFS serialize using level order then deserialize by walking the queue and assigning children by index.

Time: O(n)
Space: O(n)

function serialize(root){if(!root)return '';const q=[root],r=[];while(q.length){const n=q.shift();if(n){r.push(n.val);q.push(n.left,n.right);}else r.push('N');}return r.join(',');}
function deserialize(s){if(!s)return null;const v=s.split(',');const root=new TreeNode(+v[0]);const q=[root];let i=1;while(q.length){const n=q.shift();if(v[i]!=='N'){n.left=new TreeNode(+v[i]);q.push(n.left);}i++;if(v[i]!=='N'){n.right=new TreeNode(+v[i]);q.push(n.right);}i++;}return root;}

Tradeoff:

2. Pre-order with null sentinels

DFS pre-order serialize each node and emit a sentinel ('N') for nulls. Deserialize consumes the queue recursively, recreating the same shape.

Time: O(n)
Space: O(n)

function serialize(root) {
  const out = [];
  const dfs = (n) => {
    if (!n) { out.push('N'); return; }
    out.push(n.val);
    dfs(n.left); dfs(n.right);
  };
  dfs(root);
  return out.join(',');
}
function deserialize(data) {
  const tokens = data.split(',');
  let i = 0;
  const build = () => {
    if (tokens[i] === 'N') { i++; return null; }
    const node = new TreeNode(+tokens[i++]);
    node.left = build();
    node.right = build();
    return node;
  };
  return build();
}

Tradeoff:

Byju's-specific tips

Byju's K-12 lesson trees are persisted to JSON daily, so frame serialize/deserialize as 'curriculum snapshot transport' to land bonus signal.

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