Caeser cipher (trekhleb#517)

* added ceaserCipher algorithm

* added ceaserCipher algorithm

* fixed a typo

Author: Gifted-s

src/algorithms/string/caeserCipher/README.md

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+# caeserCipher Algorithm
+
+caeserCipher Algorithm is a type of substitution algorithm in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on
+
+## Complexity
+
+- **Time:** `O(|n|)`
+- **Space:** `O(|n|)`
+## Example
+
+The the following string `abcde` which is shifted by 1 will change to `bcdef`
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Caesar_cipher)

src/algorithms/string/caeserCipher/__test__/caeserCipher.test.js

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+import caeserCipher from '../caeserCipher';
+
+describe('caeserCipher', () => {
+    it('should subtitute each character by shifting up the alphabet by a given integer', () => {
+
+
+        expect(caeserCipher('abcd', 1)).toBe('bcde');
+    });
+    
+
+    it('should wrap back to the beginning of the alphabet if it shifts more than the 26 english alphabets', () => {
+ 
+
+        expect(caeserCipher('xyz', 1)).toBe('yza');
+    });
+    it('should only shift letters and ignore non-alphabetic characters', () => {
+
+        expect(caeserCipher('gurer ner 9 qbtf!', 13)).toBe('there are 9 dogs!');
+    });
+
+    it('should ignore case sensitivity', () => {
+
+        expect(caeserCipher('ABCD', 13)).toBe('bcde');
+    });
+    
+ 
+})

src/algorithms/string/caeserCipher/caeserCipher.js

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+/**
+ * @param {string} string
+ * @param {number} shift
+ * @return {string}  result
+ */
+
+export default function caeserCipher(string, shift) {
+    // convert all alphabets in english language to an array
+    const alphabetArr = "abcdefghijklmnopqrstuvwxyz".split("")
+    // converting all the alphabets in  string to lowercase
+    string = string.toLowerCase()
+
+    let result = ""
+    for (let i = 0; i < string.length; i++) {
+        const currentChar = string[i]
+        // checking index of character in the english alphabets
+        const idx = alphabetArr.indexOf(currentChar)
+        // skip character if the character is not an alphabet
+        if (idx === -1) {
+            result += currentChar
+            continue;
+        }
+        //wrap up index, incase the next shift is beyond the 26th character
+        const encodedIdx = (idx + shift) % 26
+        result += alphabetArr[encodedIdx]
+
+    }
+    // return the result  of the shifted string
+    return result
+}