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2227. Encrypt and Decrypt Strings

Description

You are given a character array keys containing unique characters and a string array values containing strings of length 2. You are also given another string array dictionary that contains all permitted original strings after decryption. You should implement a data structure that can encrypt or decrypt a 0-indexed string.

A string is encrypted with the following process:

  1. For each character c in the string, we find the index i satisfying keys[i] == c in keys.
  2. Replace c with values[i] in the string.

Note that in case a character of the string is not present in keys, the encryption process cannot be carried out, and an empty string "" is returned.

A string is decrypted with the following process:

  1. For each substring s of length 2 occurring at an even index in the string, we find an i such that values[i] == s. If there are multiple valid i, we choose any one of them. This means a string could have multiple possible strings it can decrypt to.
  2. Replace s with keys[i] in the string.

Implement the Encrypter class:

  • Encrypter(char[] keys, String[] values, String[] dictionary) Initializes the Encrypter class with keys, values, and dictionary.
  • String encrypt(String word1) Encrypts word1 with the encryption process described above and returns the encrypted string.
  • int decrypt(String word2) Returns the number of possible strings word2 could decrypt to that also appear in dictionary.

Β 

Example 1:

Input
["Encrypter", "encrypt", "decrypt"]
[[['a', 'b', 'c', 'd'], ["ei", "zf", "ei", "am"], ["abcd", "acbd", "adbc", "badc", "dacb", "cadb", "cbda", "abad"]], ["abcd"], ["eizfeiam"]]
Output
[null, "eizfeiam", 2]

Explanation
Encrypter encrypter = new Encrypter([['a', 'b', 'c', 'd'], ["ei", "zf", "ei", "am"], ["abcd", "acbd", "adbc", "badc", "dacb", "cadb", "cbda", "abad"]);
encrypter.encrypt("abcd"); // return "eizfeiam". 
Β                           // 'a' maps to "ei", 'b' maps to "zf", 'c' maps to "ei", and 'd' maps to "am".
encrypter.decrypt("eizfeiam"); // return 2. 
                              // "ei" can map to 'a' or 'c', "zf" maps to 'b', and "am" maps to 'd'. 
                              // Thus, the possible strings after decryption are "abad", "cbad", "abcd", and "cbcd". 
                              // 2 of those strings, "abad" and "abcd", appear in dictionary, so the answer is 2.

Β 

Constraints:

  • 1 <= keys.length == values.length <= 26
  • values[i].length == 2
  • 1 <= dictionary.length <= 100
  • 1 <= dictionary[i].length <= 100
  • All keys[i] and dictionary[i] are unique.
  • 1 <= word1.length <= 2000
  • 2 <= word2.length <= 200
  • All word1[i] appear in keys.
  • word2.length is even.
  • keys, values[i], dictionary[i], word1, and word2 only contain lowercase English letters.
  • At most 200 calls will be made to encrypt and decrypt in total.

Solutions

Solution 1: Hash Table

We use a hash table \(\textit{mp}\) to record the encryption result of each character, and another hash table \(\textit{cnt}\) to record the number of occurrences of each encryption result.

In the constructor, we traverse \(\textit{keys}\) and \(\textit{values}\), storing each character and its corresponding encryption result in \(\textit{mp}\). Then, we traverse \(\textit{dictionary}\) to count the occurrences of each encryption result. The time complexity is \(O(n + m)\), where \(n\) and \(m\) are the lengths of \(\textit{keys}\) and \(\textit{dictionary}\), respectively.

In the encryption function, we traverse each character of the input string \(\textit{word1}\), look up its encryption result, and concatenate them. If a character does not have a corresponding encryption result, it means encryption is not possible, and we return an empty string. The time complexity is \(O(k)\), where \(k\) is the length of \(\textit{word1}\).

In the decryption function, we directly return the count of \(\textit{word2}\) in \(\textit{cnt}\). The time complexity is \(O(1)\).

The space complexity is \(O(n + m)\).

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class Encrypter:
    def __init__(self, keys: List[str], values: List[str], dictionary: List[str]):
        self.mp = dict(zip(keys, values))
        self.cnt = Counter(self.encrypt(v) for v in dictionary)

    def encrypt(self, word1: str) -> str:
        res = []
        for c in word1:
            if c not in self.mp:
                return ''
            res.append(self.mp[c])
        return ''.join(res)

    def decrypt(self, word2: str) -> int:
        return self.cnt[word2]


# Your Encrypter object will be instantiated and called as such:
# obj = Encrypter(keys, values, dictionary)
# param_1 = obj.encrypt(word1)
# param_2 = obj.decrypt(word2)

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class Encrypter {
    private Map<Character, String> mp = new HashMap<>();
    private Map<String, Integer> cnt = new HashMap<>();

    public Encrypter(char[] keys, String[] values, String[] dictionary) {
        for (int i = 0; i < keys.length; ++i) {
            mp.put(keys[i], values[i]);
        }
        for (String w : dictionary) {
            cnt.merge(encrypt(w), 1, Integer::sum);
        }
    }

    public String encrypt(String word1) {
        StringBuilder sb = new StringBuilder();
        for (char c : word1.toCharArray()) {
            if (!mp.containsKey(c)) {
                return "";
            }
            sb.append(mp.get(c));
        }
        return sb.toString();
    }

    public int decrypt(String word2) {
        return cnt.getOrDefault(word2, 0);
    }
}

/**
 * Your Encrypter object will be instantiated and called as such:
 * Encrypter obj = new Encrypter(keys, values, dictionary);
 * String param_1 = obj.encrypt(word1);
 * int param_2 = obj.decrypt(word2);
 */

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class Encrypter {
public:
    unordered_map<string, int> cnt;
    unordered_map<char, string> mp;

    Encrypter(vector<char>& keys, vector<string>& values, vector<string>& dictionary) {
        for (int i = 0; i < keys.size(); ++i) {
            mp[keys[i]] = values[i];
        }
        for (auto v : dictionary) {
            cnt[encrypt(v)]++;
        }
    }

    string encrypt(string word1) {
        string res = "";
        for (char c : word1) {
            if (!mp.count(c)) {
                return "";
            }
            res += mp[c];
        }
        return res;
    }

    int decrypt(string word2) {
        return cnt[word2];
    }
};

/**
 * Your Encrypter object will be instantiated and called as such:
 * Encrypter* obj = new Encrypter(keys, values, dictionary);
 * string param_1 = obj->encrypt(word1);
 * int param_2 = obj->decrypt(word2);
 */

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type Encrypter struct {
    mp  map[byte]string
    cnt map[string]int
}

func Constructor(keys []byte, values []string, dictionary []string) Encrypter {
    mp := map[byte]string{}
    cnt := map[string]int{}
    for i, k := range keys {
        mp[k] = values[i]
    }
    e := Encrypter{mp, cnt}
    for _, v := range dictionary {
        e.cnt[e.Encrypt(v)]++
    }
    return e
}

func (this *Encrypter) Encrypt(word1 string) string {
    var ans strings.Builder
    for _, c := range word1 {
        if v, ok := this.mp[byte(c)]; ok {
            ans.WriteString(v)
        } else {
            return ""
        }
    }
    return ans.String()
}

func (this *Encrypter) Decrypt(word2 string) int {
    return this.cnt[word2]
}

/**
 * Your Encrypter object will be instantiated and called as such:
 * obj := Constructor(keys, values, dictionary);
 * param_1 := obj.Encrypt(word1);
 * param_2 := obj.Decrypt(word2);
 */

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class Encrypter {
    private mp: Map<string, string> = new Map();
    private cnt: Map<string, number> = new Map();

    constructor(keys: string[], values: string[], dictionary: string[]) {
        for (let i = 0; i < keys.length; i++) {
            this.mp.set(keys[i], values[i]);
        }
        for (const w of dictionary) {
            const encrypted = this.encrypt(w);
            if (encrypted !== '') {
                this.cnt.set(encrypted, (this.cnt.get(encrypted) || 0) + 1);
            }
        }
    }

    encrypt(word: string): string {
        let res = '';
        for (const c of word) {
            if (!this.mp.has(c)) {
                return '';
            }
            res += this.mp.get(c);
        }
        return res;
    }

    decrypt(word: string): number {
        return this.cnt.get(word) || 0;
    }
}

/**
 * Your Encrypter object will be instantiated and called as such:
 * const obj = new Encrypter(keys, values, dictionary);
 * const param_1 = obj.encrypt(word1);
 * const param_2 = obj.decrypt(word2);
 */

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