[Array] Add a solution to Longest Substring with At Most Two Distinct Characters

Author: soapyigu

Array/LongestSubstringMostTwoDistinctCharacters.swift

@@ -0,0 +1,40 @@
+/**
+ * Question Link: https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/
+ * Primary idea: Slding window, use dictionary to check substring is valid or not, and 
+ 				 note to handle the end of string edge case
+ *
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ */
+
+ class LongestSubstringMostTwoDistinctCharacters {
+    func lengthOfLongestSubstringTwoDistinct(_ s: String) -> Int {
+        var start = 0, longest = 0, charFreq = [Character: Int]()
+        let sChars = Array(s)
+        
+        for (i, char) in sChars.enumerated() {
+            if let freq = charFreq[char] {
+                charFreq[char] = freq + 1
+            } else {
+               if charFreq.count == 2 {
+                    longest = max(longest, i - start)
+                    
+                    while charFreq.count == 2 {
+                        let charStart = sChars[start]
+                        charFreq[charStart]! -= 1
+                        
+                        if charFreq[charStart] == 0 {
+                            charFreq[charStart] = nil
+                        }
+                        
+                        start += 1
+                    }   
+                }
+                
+                charFreq[char] = 1
+            }
+        }
+        
+        return max(longest, sChars.count - start)
+    }
+}

README.md

@@ -28,7 +28,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 235 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 236 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
@@ -70,9 +70,11 @@
 [Game of Life](https://leetcode.com/problems/game-of-life/)| [Swift](./Array/GameLife.swift)| Medium| O(n)| O(1)|
 [Task Scheduler](https://leetcode.com/problems/task-scheduler/)| [Swift](./Array/TaskScheduler.swift)| Medium| O(nlogn)| O(n)|
 [Sliding Window Maximum ](https://leetcode.com/problems/sliding-window-maximum/)| [Swift](./Array/SlidingWindowMaximum.swift)| Hard| O(n)| O(n)|
+[Longest Substring with At Most Two Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/)| [Swift](./Array/LongestSubstringMostTwoDistinctCharacters.swift)| Hard| O(n)| O(n)|	
 [Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/)| [Swift](./Array/LongestConsecutiveSequence.swift)| Hard| O(n)| O(n)|
 
 
+
 ## String
 | Title | Solution | Difficulty | Time | Space |
 | ----- | -------- | ---------- | ---- | ----- |
@@ -633,7 +635,7 @@
 |	[Swift](./Search/FindPeakElement.swift)	|	162	|	[Find Peak Element](https://oj.leetcode.com/problems/find-peak-element/)	| Medium	|
 |	[Swift](./String/OneEditDistance.swift)	|	161	|	[One Edit Distance](https://oj.leetcode.com/problems/one-edit-distance/)♥	| Medium	|
 |		|	160	|	[Intersection of Two Linked Lists](https://oj.leetcode.com/problems/intersection-of-two-linked-lists/)	| Easy	|
-|		|	159	|	[Longest Substring with At Most Two Distinct Characters](https://oj.leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/) ♥	| Hard	|
+|	[Swift](./Array/LongestSubstringMostTwoDistinctCharacters.swift)	|	159	|	[Longest Substring with At Most Two Distinct Characters](https://oj.leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/) ♥	| Hard	|
 |		|	158	|	[Read N Characters Given Read4 II - Call multiple times](https://oj.leetcode.com/problems/read-n-characters-given-read4-ii-call-multiple-times/) ♥	| Hard	|
 |		|	157	|	[Read N Characters Given Read4](https://oj.leetcode.com/problems/read-n-characters-given-read4/) ♥	| Easy	|
 |	[Swift](./Tree/BinaryTreeUpsideDown) |	156	|	[Binary Tree Upside Down](https://oj.leetcode.com/problems/binary-tree-upside-down/) ♥	| Medium	|