### Pramp Secret Message Problem

An infamous gang of cyber criminals named “The Gray Cyber Mob”, which is behind many hacking attacks and drug trafficking, has recently become a target for the FBI. After intercepting some of their messages, which looked like complete nonsense, the agency learned that they indeed encrypt their messages, and studied their method of encryption.

Their messages consist of lowercase latin letters only, and every word is encrypted separately as follows:

Convert every letter to its ASCII value. Add `1` to the first letter, and then for every letter from the second one to the last one, add the value of the previous letter. Subtract `26` from every letter until it is in the range of lowercase letters `a-z` in ASCII. Convert the values back to letters.

For instance, to encrypt the word `“crime”`

The FBI needs an efficient method to decrypt messages. Write a function named `decrypt(word)` that receives a string that consists of small latin letters only, and returns the decrypted word.

Explain your solution and analyze its time and space complexities.

Examples:

```input:  word = "dnotq"
output: "crime"

input:  word = "flgxswdliefy"
output: "encyclopedia"
```

Since the function should be used on messages with many words, make sure the function is as efficient as possible in both time and space. Explain the correctness of your function, and analyze its asymptotic runtime and space complexity.

Note: Most programing languages have built-in methods of converting letters to ASCII values and vica versa. You may search the internet for the appropriate method.

Constraints:

• [time limit] 5000ms
• [input] string `word`
• The ASCII value of every char is in the range of lowercase letters `a-z`.
• [output] string

### Solution

``````import Foundation

func decrypt(word: String) -> String {
var ascii = word.map { Int(\$0.asciiValue!) }

let count = ascii.count

var prev = ascii[0]
for i in 1..<count {
let temp = ascii[i]
ascii[i] = ascii[i] - prev
prev = temp
}

ascii[0] -= 1

for i in 0..<count {
while ascii[i] < 96 {
ascii[i] += 26
}
}

return String(ascii.map {
Character(UnicodeScalar(UInt8(\$0)))
})
}``````

### One Comment

• David Inga says:
``````import Foundation

func decrypt(word: String) -> String {
var ascii = word.map { Int(\$0.asciiValue!) }

let count = ascii.count

var prev = ascii[0]
for i in 1..``````

Designed by David Inga in California.