[String] Add a solution to String Compression

Author: soapyigu

README.md

@@ -28,7 +28,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 257 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
+[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 258 completed solutions. Note: questions with ♥ mark means that you have to **Subscript to premium membership** of LeetCode to unlock them. Thank you for great contributions from [CharleneJiang](https://github.com/CharleneJiang), [ReadmeCritic](https://github.com/ReadmeCritic), [demonkoo](https://github.com/demonkoo), [DaiYue](https://github.com/DaiYue), [Quaggie](https://github.com/Quaggie) and [jindulys](https://github.com/jindulys).
 
 
 ## Array
@@ -94,6 +94,7 @@
 [Reverse String II](https://leetcode.com/problems/reverse-string-ii/)| [Swift](./String/ReverseStringII.swift)| Easy| O(n)| O(n)|
 [Reverse Vowels of a String](https://leetcode.com/problems/reverse-vowels-of-a-string/)| [Swift](./String/ReverseVowelsOfAString.swift)| Easy| O(n)| O(n)|
 [Length of Last Word](https://leetcode.com/problems/length-of-last-word/)| [Swift](./String/AddStrings.swift)| Easy| O(n)| O(n)|
+[String Compression](https://leetcode.com/problems/string-compression/)| [Swift](./String/StringCompression.swift)| Easy| O(n)| O(1)|
 [Add Strings](https://leetcode.com/problems/add-strings/)| [Swift](./String/LengthLastWord.swift)| Easy| O(n)| O(1)|
 [Multiply Strings](https://leetcode.com/problems/multiply-strings/)| [Swift](./String/MultiplyStrings.swift)| Medium| O(n)| O(1)|
 [Palindrome Permutation](https://leetcode.com/problems/palindrome-permutation/)| [Swift](./String/PalindromePermutation.swift)| Easy| O(n)| O(n)|

String/StringCompression.swift

@@ -0,0 +1,38 @@
+/**
+ * Question Link: https://leetcode.com/problems/string-compression/
+ * Primary idea: Compare two consecutive characters, set character at index and count of it afterwards
+ * 
+ * Note: do not forget to handle edge case when hit the end of array
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+ class StringCompression {
+    func compress(_ chars: inout [Character]) -> Int {
+        var index = 0, currentCount = 0
+        
+        for i in 0..<chars.count {
+            currentCount += 1
+            
+            if i + 1 == chars.count || chars[i] != chars[i + 1] {
+                chars[index] = chars[i]
+                
+                if currentCount != 1 {
+                    chars.replaceSubrange(index + 1...currentCount.length + index, with: Array(String(currentCount)))
+                }
+                
+                index += currentCount == 1 ? 1 : 1 + currentCount.length
+                currentCount = 0
+            }
+        }
+        
+        return index
+    }
+}
+
+extension Int {
+    var length: Int {
+        return String(self).count
+    }
+}