CPMagical CP Library

Problem solution · Java

Encrypt and Decrypt Strings

Encrypt and Decrypt Strings: a Java solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Hash-based lookup
Source
walkccc LeetCode Solutions
Length
24 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Hash-based lookup

For Encrypt and Decrypt Strings, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 24 lines of Java from the credited upstream file 2227-2.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 3 loop blocks detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeEncrypt and Decrypt Strings · JavaJava
Use this to learn the idea, then write your own version.
class Encrypter {  public Encrypter(char[] keys, String[] values, String[] dictionary) {    for (int i = 0; i < keys.length; ++i)      keyToValue.put(keys[i], values[i]);     for (final String word : dictionary)      encryptedCount.merge(encrypt(word), 1, Integer::sum);  }   public String encrypt(String word1) {    StringBuilder sb = new StringBuilder();    for (final char c : word1.toCharArray())      sb.append(keyToValue.get(c));    return sb.toString();  }   public int decrypt(String word2) {    return encryptedCount.getOrDefault(word2, 0);  }   private Map<Character, String> keyToValue = new HashMap<>();  private Map<String, Integer> encryptedCount = new HashMap<>();}

Keep practising

Did this explanation save you time? I'm a Grade 11 student building this free library to make difficult algorithms easier to understand.

Buy me a coffee ↗