Typorithm · hard

Serialize & Deserialize

serialize-deserializetreespreordernull-markercodec

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Full lesson · 8 learning steps Open any exercise for focused practice

Step 1 of 8 · intro card

Learn Serialize & Deserialize

Trees

Pre-order DFS emits each node with '#' for nulls; deserialize consumes the same token stream to rebuild the exact tree.

Serializing must capture both values and shape, so null children get an explicit '#' marker.

A pre-order DFS writes node, then left subtree, then right subtree, into a comma-joined string.

Deserialize consumes tokens in that same order, recursing to rebuild left and right, recreating the original tree.

Pre-order with null markers.

'#' encodes an empty child.

Rebuild reads tokens in the same order.

function serialize(root) {
  const parts = [];
  function dfs(node) {
    if (node === null) {
      parts.push("#");
      return;
    }
    parts.push(String(node.val));
    dfs(node.left);
    dfs(node.right);
  }
  dfs(root);
  return parts.join(",");
}

function deserialize(data) {
  const parts = data.split(",");
  let i = 0;
  function build() {
    const token = parts[i];
    i += 1;
    if (token === "#") {
      return null;
    }
    const node = { val: Number(token), left: null, right: null };
    node.left = build();
    node.right = build();
    return node;
  }
  return build();
}

Read the implementation

Connect each part of the code to the concept before you type.

1. Null marker

A sentinel for empty children makes the encoding unambiguous.

parts.push("#")

2. Pre-order emit

Record the node before recursing, capturing structure.

parts.push(String(node.val));
    dfs(node.left);
    dfs(node.right)

3. Consume token

Rebuild reads tokens in the same pre-order sequence.

const token = parts[i];
    i += 1

4. Rebuild children

Recursion reconstructs left then right, mirroring serialize.

node.left = build();
    node.right = build()

A complete learning loop

Understand

Read the concept map and follow the code breakdown.

Reconstruct

Type the implementation, repair mistakes and work through every recall exercise.

Verify

Use the final check without hints; revisit weak concepts in short practice sessions.