[Array] Add a solution to Smallest Range

Author: soapyigu

Diff for: Array/SmallestRange.swift

@@ -0,0 +1,51 @@
+/**
+ * Question Link: https://leetcode.com/problems/smallest-range/
+ * Primary idea: Merge K lists + Minimum Window Substring
+ * Time Complexity: O(nm), Space Complexity: O(nm)
+ *
+ */
+
+ class SmallestRange {
+    func smallestRange(_ nums: [[Int]]) -> [Int] {        
+        let mergedNums = merge(nums)
+        
+        var left = 0, diff = Int.max, res = [Int]()
+        var numsIndexFreq = Dictionary((0..<nums.count).map { ($0, 0) }, uniquingKeysWith: +), count = 0
+        
+        for (right, numIndex) in mergedNums.enumerated() {
+            if numsIndexFreq[numIndex.1]! == 0 {
+                count += 1
+            }
+    
+            numsIndexFreq[numIndex.1]! += 1
+            
+            while count == nums.count {
+                if diff > mergedNums[right].0 - mergedNums[left].0 {
+                    diff = mergedNums[right].0 - mergedNums[left].0
+                    res = [mergedNums[left], mergedNums[right]].map { $0.0 }
+                }
+                
+                if numsIndexFreq[mergedNums[left].1]! == 1 {
+                    count -= 1
+                }
+                numsIndexFreq[mergedNums[left].1]! -= 1
+                
+                left += 1
+            }
+        }
+        
+        return res
+    }
+    
+    fileprivate func merge(_ nums: [[Int]]) -> [(Int, Int)] {
+        var res = [(Int, Int)]()
+        
+        for (numsIndex, nums) in nums.enumerated() {
+            for num in nums {
+                res.append((num, numsIndex))
+            }
+        }
+        
+        return res.sorted { return $0.0 < $1.0 }
+    }
+}

Diff for: README.md

@@ -27,7 +27,7 @@
 * [Microsoft](#microsoft)
 
 ## Progress
-[Problem Status](#problem-status) shows the latest progress to all 800+ questions. Currently we have 229 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 230 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
@@ -54,8 +54,9 @@
 [Summary Ranges](https://leetcode.com/problems/summary-ranges/)| [Swift](./Array/SummaryRanges.swift)| Medium| O(n)| O(n)|
 [Shortest Word Distance](https://leetcode.com/problems/shortest-word-distance/)| [Swift](./Array/ShortestWordDistance.swift)| Easy| O(n)| O(1)|
 [Shortest Word Distance III](https://leetcode.com/problems/shortest-word-distance-iii/)| [Swift](./Array/ShortestWordDistanceIII.swift)| Medium| O(n)| O(1)|
-[Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum/)| [Swift](./Array/MinimumSizeSubarraySum.swift)| Easy| O(n)| O(1)|
-[Maximum Size Subarray Sum Equals k](https://leetcode.com/problems/maximum-size-subarray-sum-equals-k/)| [Swift](./Array/MaximumSizeSubarraySumEqualsK.swift)| Easy| O(n)| O(n)|
+[Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum/)| [Swift](./Array/MinimumSizeSubarraySum.swift)| Medium| O(n)| O(1)|
+[Maximum Size Subarray Sum Equals k](https://leetcode.com/problems/maximum-size-subarray-sum-equals-k/)| [Swift](./Array/MaximumSizeSubarraySumEqualsK.swift)| Medium| O(n)| O(n)|
+[Smallest Range](https://leetcode.com/problems/smallest-range/)| [Swift](./Array/SmallestRange.swift)| Hard | O(nm)| O(nm)|
 [Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/)| [Swift](./Array/ProductExceptSelf.swift)| Medium| O(n)| O(n)|
 [Rotate Array](https://leetcode.com/problems/rotate-array/)| [Swift](./Array/RotateArray.swift)| Easy| O(n)| O(1)|
 [Rotate Image](https://leetcode.com/problems/rotate-image/)| [Swift](./Array/RotateImage.swift)| Medium| O(n^2)| O(1)|