Encode and Decode Strings

The problem

Design an encoding scheme that converts a list of strings into a single string and a decoding scheme that reconstructs the original list from the encoded string.

Any delimiter choice can appear legitimately inside the payload strings. The only invariant that remains uncorrupted is the exact byte length of the upcoming token. Prefixing each item with its integer character count followed by an unambiguous separator turns the stream into a sequence of self-describing segments where content can contain any symbol.

Full solution

Length-prefixed framing: each item becomes length + '#' + content, so the decoder reads the count and jumps exactly that many characters — any symbol, including '#', can appear in the payload with no escaping.

def encode(strs: list[str]) -> str:
    # length-prefixed framing: "<len>#<chars>" per item, so '#' inside content is safe
    return "".join(f"{len(s)}#{s}" for s in strs)


def decode(s: str) -> list[str]:
    out: list[str] = []
    i = 0
    while i < len(s):
        j = s.index("#", i)          # digits between i and j are the length
        k = int(s[i:j])
        out.append(s[j + 1:j + 1 + k])  # jump exactly k chars, never search for '#'
        i = j + 1 + k
    return out

function encode(strs) {
  // length-prefixed framing: "<len>#<chars>" per item, so '#' inside content is safe
  return strs.map((s) => `${s.length}#${s}`).join("");
}

function decode(s) {
  const out = [];
  let i = 0;
  while (i < s.length) {
    const j = s.indexOf("#", i); // digits between i and j are the length
    const k = Number(s.slice(i, j));
    out.push(s.slice(j + 1, j + 1 + k)); // jump exactly k chars, never search for '#'
    i = j + 1 + k;
  }
  return out;
}

The theory behind it

Strings — the ground this problem stands on. All Strings problems

What Strings is

A string is an ordered necklace of text characters, like letters printed along a ribbon of paper. Each character sits at an exact numeric slot, holding a glyph such as a letter, punctuation mark, or digit. In many programming languages, ribbons cannot be edited after creation, meaning changing a single character requires pressing an entirely new ribbon from scratch.

When to reach for it

Reach for string techniques when inputs consist of words, DNA sequences, serialized data formats, or sentences. Clues include questions testing palindromes, anagram matches, substring patterns, parenthesis balancing, or character frequency counts. Whenever an algorithm asks to transform capitalization, parse structured tokens, or compute edits between two phrases, string representations are the core subject.

How the pattern works

Think of characters as small integer codes ranging across standard character sets. Frequency tables with fixed sizes often replace heavy hash maps when tallying occurrences. For search tasks, maintain rolling state using character indices or sliding borders. When building output text through repeated appends, accumulate pieces inside a mutable list or string builder rather than concatenating strings directly, avoiding quadratic copy overhead.

What each operation costs

Which roles need this problem

Strings is a core topic for these 13 roles — if you're targeting one of them, this problem is early in your path, not optional.

Secondary for 7 more roles, including Data Engineer, Data Analyst, Embedded / Firmware Engineer.