feat: add string compression solution

Author: sandrig

README.md

@@ -8,3 +8,9 @@
 | 2   | [Move Zeroes](https://leetcode.com/problems/move-zeroes/)           | [TypeScript](https://github.com/sandrig/leetcode/blob/master/typescript/src/moveZeroes/README.md)      | _O(n)_ | _O(1)_ | Array, Two Pointers  |
 | 3   | [Valid Palindrome](https://leetcode.com/problems/valid-palindrome/) | [TypeScript](https://github.com/sandrig/leetcode/blob/master/typescript/src/validPalindrome/README.md) | _O(n)_ | _O(n)_ | String, Two Pointers |
 | 4   | [Summary Ranges](https://leetcode.com/problems/summary-ranges/)     | [TypeScript](https://github.com/sandrig/leetcode/blob/master/typescript/src/summaryRanges/README.md)   | _O(n)_ | _O(n)_ | Array                |
+
+## Medium
+
+| #   | Problem                                                                 | Solution                                                                                                 | Time   | Space  | Tag                  |
+| --- | ----------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------- | ------ | ------ | -------------------- |
+| 1   | [String Compression](https://leetcode.com/problems/string-compression/) | [TypeScript](https://github.com/sandrig/leetcode/blob/master/typescript/src/stringCompression/README.md) | _O(n)_ | _O(n)_ | Two Pointers, String |

typescript/src/stringCompression/README.md

@@ -0,0 +1,80 @@
+# [String Compression](https://leetcode.com/problems/string-compression/)
+
+## Description
+
+Given an array of characters **chars**, compress it using the following algorithm:
+
+Begin with an empty string **s**. For each group of consecutive repeating characters in **chars**:
+
+- If the group's length is **1**, append the character to **s**.
+- Otherwise, append the character followed by the group's length.
+
+The compressed string **s** should not be returned separately, but instead, be stored in the input character array **chars**. Note that group lengths that are **10** or longer will be split into multiple characters in **chars**.
+
+After you are done modifying the input array, return the new length of the array.
+
+You must write an algorithm that uses only constant extra space.
+
+**Example 1:**
+
+```
+Input: chars = ["a","a","b","b","c","c","c"]
+Output: Return 6, and the first 6 characters of the input array should be: ["a","2","b","2","c","3"]
+Explanation: The groups are "aa", "bb", and "ccc". This compresses to "a2b2c3".
+```
+
+**Example 2:**
+
+```
+Input: chars = ["a"]
+Output: Return 1, and the first character of the input array should be: ["a"]
+Explanation: The only group is "a", which remains uncompressed since it's a single character.
+```
+
+**Example 3:**
+
+```
+Input: chars = ["a","b","b","b","b","b","b","b","b","b","b","b","b"]
+Output: Return 4, and the first 4 characters of the input array should be: ["a","b","1","2"].
+Explanation: The groups are "a" and "bbbbbbbbbbbb". This compresses to "ab12".
+```
+
+## Solution
+
+```typescript
+function compress(chars: string[]): number {
+  let writeIndex = 0; // Index to write compressed characters
+  let readIndex = 0; // Index to read source characters
+
+  while (readIndex < chars.length) {
+    const currentChar = chars[readIndex];
+    let count = 0;
+
+    // Counting the number of repeated characters
+    while (readIndex < chars.length && chars[readIndex] === currentChar) {
+      readIndex++;
+      count++;
+    }
+
+    // Write a compressed character
+    chars[writeIndex] = currentChar;
+    writeIndex++;
+
+    // If the character is repeated more than once, record the number of repetitions
+    if (count > 1) {
+      for (const digit of count.toString().split("")) {
+        chars[writeIndex] = digit;
+        writeIndex++;
+      }
+    }
+  }
+
+  // Returning the new length of the compressed string
+  return writeIndex;
+}
+```
+
+## Complexity Analysis
+
+- Time complexity: _O(n)_
+- Space complexity: _O(n)_

typescript/src/stringCompression/stringCompression.test.ts

@@ -0,0 +1,66 @@
+import { compress } from "./stringCompression";
+
+describe("String Compression", () => {
+  it("should compress the string with repeated characters", () => {
+    const chars: string[] = ["a", "a", "b", "b", "c", "c", "c"];
+    const compressedLength = compress(chars);
+    expect(compressedLength).toBe(6);
+    expect(chars.slice(0, compressedLength)).toEqual([
+      "a",
+      "2",
+      "b",
+      "2",
+      "c",
+      "3",
+    ]);
+  });
+
+  it("should return the original string if it cannot be compressed", () => {
+    const chars: string[] = ["a"];
+    const compressedLength = compress(chars);
+    expect(compressedLength).toBe(1);
+    expect(chars.slice(0, compressedLength)).toEqual(["a"]);
+  });
+
+  it("should handle multiple repeating characters", () => {
+    const chars: string[] = [
+      "a",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+      "b",
+    ];
+    const compressedLength = compress(chars);
+    expect(compressedLength).toBe(4);
+    expect(chars.slice(0, compressedLength)).toEqual(["a", "b", "1", "2"]);
+  });
+
+  it("should handle alternating characters", () => {
+    const chars: string[] = ["a", "a", "a", "b", "b", "a", "a"];
+    const compressedLength = compress(chars);
+    expect(compressedLength).toBe(6);
+    expect(chars.slice(0, compressedLength)).toEqual([
+      "a",
+      "3",
+      "b",
+      "2",
+      "a",
+      "2",
+    ]);
+  });
+
+  it("should handle an empty string", () => {
+    const chars: string[] = [];
+    const compressedLength = compress(chars);
+    expect(compressedLength).toBe(0);
+    expect(chars.slice(0, compressedLength)).toEqual([]);
+  });
+});

typescript/src/stringCompression/stringCompression.ts

@@ -0,0 +1,30 @@
+export function compress(chars: string[]): number {
+  let writeIndex = 0; // Index to write compressed characters
+  let readIndex = 0; // Index to read source characters
+
+  while (readIndex < chars.length) {
+    const currentChar = chars[readIndex];
+    let count = 0;
+
+    // Counting the number of repeated characters
+    while (readIndex < chars.length && chars[readIndex] === currentChar) {
+      readIndex++;
+      count++;
+    }
+
+    // Write a compressed character
+    chars[writeIndex] = currentChar;
+    writeIndex++;
+
+    // If the character is repeated more than once, record the number of repetitions
+    if (count > 1) {
+      for (const digit of count.toString().split("")) {
+        chars[writeIndex] = digit;
+        writeIndex++;
+      }
+    }
+  }
+
+  // Returning the new length of the compressed string
+  return writeIndex;
+}