# LeetCode 271: Encode And Decode Strings — Step-by-Step Visual Trace

**Medium** — String | Array | Design | Parsing

## The Problem

Design an algorithm to encode a list of strings into a single string, then decode it back to the original list of strings. The encoded string should handle edge cases like empty strings and strings containing special characters.

## Approach

The encoding uses a length-prefix format where each string is prefixed with its length followed by a delimiter '#'. During decoding, we parse the length, skip the delimiter, then extract exactly that many characters to reconstruct each original string.

**Time:** O(n) · **Space:** O(n)

## Code

```python
class Codec:
    def encode(self, strs: List[str]) -> str:
        encoded = ""
        for s in strs:
            encoded += str(len(s)) + "#" + s
        return encoded

    def decode(self, s: str) -> List[str]:
        decoded = []
        i = 0
        while i < len(s):
            delimiter_pos = s.find("#", i)
            size = int(s[i:delimiter_pos])
            start_pos = delimiter_pos + 1
            end_pos = start_pos + size
            decoded.append(s[start_pos:end_pos])
            i = end_pos
        return decoded
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0271_encode-and-decode-strings)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0271_encode-and-decode-strings) and step through every line.

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