Update Merge Intervals solution and difficulty

DP/MaximumSubarray.swift

@@ -6,15 +6,14 @@
  */
 
 class MaximumSubarray {
-    func maxSubArray(nums: [Int]) -> Int {
-        var max_current = nums[0]
-        var max_global = nums[0]
-
-        for i in 1..<nums.count {
-            max_current = max(max_current + nums[i], nums[i])
-            max_global = max(max_current, max_global)
+    func maxSubArray(_ nums: [Int]) -> Int {
+        var nums = nums
+        var ret = nums.removeFirst()
+        var curr = ret
+        for num in nums {
+            curr = max(num, curr + num)
+            ret = max(ret, curr)
         }
-
-        return max_global
+        return ret
     }
-}
+}

README.md

@@ -368,7 +368,7 @@
 [Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/)| [Swift](./Sort/TopKFrequentElements.swift)| Medium| O(nlogn)| O(n)|
 [Meeting Rooms](https://leetcode.com/problems/meeting-rooms/)| [Swift](./Sort/MeetingRooms.swift)| Easy| O(nlogn)| O(1)|
 [Meeting Rooms II](https://leetcode.com/problems/meeting-rooms-ii/)| [Swift](./Sort/MeetingRoomsII.swift)| Medium| O(nlogn)| O(n)|
-[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Hard| O(nlogn)| O(n)|
+[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Medium| O(nlogn)| O(n)|
 [Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)| [Swift](./Graph/AlienDictionary.swift)| Hard| O(nm)| O(nm)|
 [Kth Largest Element in an Array](https://leetcode.com/problems/kth-largest-element-in-an-array/)| [Swift](./Sort/KthLargestElementInArray.swift)| Medium| O(nlogn)| O(n)|
 [Array Partition I](https://leetcode.com/problems/array-partition-i/description/)| [Swift](./Sort/ArrayPartitionI.swift)|Easy| O(nlogn)| O(n)|
@@ -405,7 +405,7 @@
 [Number of Islands](https://leetcode.com/problems/number-of-islands/)| [Swift](./DFS/NumberofIslands.swift)| Medium| ★★★★|
 [Summary Ranges](https://leetcode.com/problems/summary-ranges/)| [Swift](./Array/SummaryRanges.swift)| Medium| ★★★★|
 [Perfect Squares](https://leetcode.com/problems/perfect-squares/)| [Swift](./DP/PerfectSquares.swift)| Medium| ★★★★|
-[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Hard| ★★★|
+[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Medium| ★★★|
 [Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| [Swift](./Stack/ValidParentheses.swift)| Easy| ★★★|
 [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)| [Swift](./Math/TrappingRainWater.swift)| Hard| ★★|
 [Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)| [Swift](./LinkedList/MergeKSortedLists.swift)| Hard| ★★|
@@ -432,7 +432,7 @@
 [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)| [Swift](./DFS/LetterCombinationsPhoneNumber.swift)| Medium| ★★★★|
 [Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)| [Swift](./LinkedList/MergeKSortedLists.swift)| Hard| ★★★★|
 [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)| [Swift](./LinkedList/ReverseLinkedList.swift)| Easy| ★★★|
-[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Hard| ★★★|
+[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Medium| ★★★|
 [Number of Islands](https://leetcode.com/problems/number-of-islands/)| [Swift](./DFS/NumberofIslands.swift)| Medium| ★★★|
 [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)| [Swift](./LinkedList/ReverseLinkedList.swift)| Easy| ★★★|
 [Expression Add Operators](https://leetcode.com/problems/expression-add-operators/)| [Swift](./DFS/ExpressionAddOperators.swift)| Hard| ★★★|
@@ -481,7 +481,7 @@
 | ----- | -------- | ---------- | --------- |
 [Maximum Subarray](https://leetcode.com/problems/maximum-subarray/)| [Swift](./DP/MaximumSubarray.swift)| Medium| ★★★★★★|
 [Pow(x, n)](https://leetcode.com/problems/isomorphic-strings/)| [Swift](./Math/Pow.swift)| Medium| ★★★★★★|
-[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Hard| ★★★★★★|
+[Merge Intervals](https://leetcode.com/problems/merge-intervals/)| [Swift](./Sort/MergeIntervals.swift)| Medium| ★★★★★★|
 [Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings/)| [Swift](./String/IsomorphicStrings.swift)| Easy| ★★★★★★|
 [Search in Rotated Sorted Array](https://leetcode.com/problems/search-in-rotated-sorted-array/)| [Swift](./Search/SearchInRotatedSortedArray.swift)| Hard| ★★★★★|
 [Search for a Range](https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array/)| [Swift](./Search/SearchForARange.swift)| Medium| ★★★★★|
@@ -833,7 +833,7 @@
 |	[Swift](./Array/SpiralMatrixII.swift)	|	59	|	[Spiral Matrix II](https://oj.leetcode.com/problems/spiral-matrix-ii/)	| Medium	|
 |	[Swift](./String/LengthLastWord.swift)	|	58	|	[Length of Last Word](https://oj.leetcode.com/problems/length-of-last-word/)	| Easy	|
 | [Swift](./Sort/InsertInterval.swift) |	57	|	[Insert Interval](https://oj.leetcode.com/problems/insert-interval/)	| Hard	|
-|	[Swift](./Sort/MergeIntervals.swift)	|	56	|	[Merge Intervals](https://oj.leetcode.com/problems/merge-intervals/)	| Hard	|
+|	[Swift](./Sort/MergeIntervals.swift)	|	56	|	[Merge Intervals](https://oj.leetcode.com/problems/merge-intervals/)	| Medium	|
 |	[Swift](./DP/JumpGame.swift) |	55	|	[Jump Game](https://oj.leetcode.com/problems/jump-game/)	| Medium	|
 |	[Swift](./Array/SpiralMatrix.swift)	|	54	|	[Spiral Matrix](https://oj.leetcode.com/problems/spiral-matrix/)	| Medium	|
 |	[Swift](./DP/MaximumSubarray.swift)	|	53	|	[Maximum Subarray](https://oj.leetcode.com/problems/maximum-subarray/)	| Medium	|

Sort/MergeIntervals.swift

@@ -1,44 +1,29 @@
 /**
  * Question Link: https://leetcode.com/problems/merge-intervals/
- * Primary idea: Sort the original intervals and then append them one by one
+ * Primary idea: Sort intervals by leading integer in ascending order and append based on their leading and trailing integers.
  * Time Complexity: O(nlogn), Space Complexity: O(n)
- *
- * Definition for an interval.
- * public class Interval {
- *   public var start: Int
- *   public var end: Int
- *   public init(_ start: Int, _ end: Int) {
- *     self.start = start
- *     self.end = end
- *   }
- * }
  */
 
 class MergeIntervals {
-    func merge(intervals: [Interval]) -> [Interval] {
-        var result = [Interval]()
-
-        let intervals = intervals.sorted {
-            if $0.start != $1.start {
-                return $0.start < $1.start
+    func merge(_ intervals: [[Int]]) -> [[Int]] {
+        guard intervals.count > 1 else { return intervals }
+        let intervals = intervals.sorted { $0[0] < $1[0] }
+        var ret: [[Int]] = []
+        var i = 1
+        var last = intervals[0]
+        while i < intervals.endIndex {
+            let curr = intervals[i]
+            if curr[0] <= last[1] {
+                if curr[1] > last[1] {
+                    last[1] = curr[1]
+                }
             } else {
-                return $0.end < $1.end
+                ret.append(last)
+                last = curr
             }
+            guard i != intervals.endIndex - 1 else { ret.append(last); break }
+            i += 1
         }
-
-        for interval in intervals {
-            guard let last = result.last else {
-                result.append(interval)
-                continue
-            }
-
-            if last.end < interval.start {
-                result.append(interval)
-            } else {
-                last.end = max(last.end, interval.end)
-            }
-        }
-
-        return result
+        return ret
     }
-}
+}