Encode and Decode Strings: the idea to remember.

The core insight

Write each string as its character length, a separator, then exactly that many characters. Decode by reading the length before consuming the payload.

Why it works

A length prefix tells the decoder exactly where a payload ends, so separators inside the payload cannot create ambiguity.

The mistake to avoid

Empty list and a list containing an empty string must remain different. Java length counts UTF-16 code units; encoding and decoding use the same convention.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

class Codec {
    String encode(List<String> words) {
        StringBuilder out = new StringBuilder();
        for (String word : words) out.append(word.length()).append('#').append(word);
        return out.toString();
    }
    List<String> decode(String encoded) {
        List<String> out = new ArrayList<>(); int i = 0;
        while (i < encoded.length()) {
            int marker = encoded.indexOf('#', i);
            int length = Integer.parseInt(encoded.substring(i, marker));
            int start = marker + 1;
            out.add(encoded.substring(start, start + length)); i = start + length;
        }
        return out;
    }
}

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