[Array] add Solution to Four Sum and Summary Ranges

Author: Yi Gu

Array/FourSum.swift

@@ -0,0 +1,59 @@
+/**
+ * Question Link: https://leetcode.com/problems/4sum/
+ * Primary idea: Sort the array, and traverse it, increment left or decrease right 
+ *               predicated on their sum is greater or not than the target
+ * Time Complexity: O(n^3), Space Complexity: O(nC4)
+ */
+
+class FourSum {
+    func fourSum(nums: [Int], _ target: Int) -> [[Int]] {
+        let nums = nums.sort({$0 < $1})
+        var threeSum = 0
+        var twoSum = 0
+        var left = 0
+        var right = 0
+        var res = [[Int]]()
+        
+        guard nums.count >= 4 else {
+            return res
+        }
+        
+        for i in 0 ..< nums.count - 3 {
+            guard i == 0 || nums[i] != nums[i - 1] else {
+                continue
+            }
+            threeSum = target - nums[i]
+            
+            for j in i + 1 ..< nums.count - 2 {
+                guard j == i + 1 || nums[j] != nums[j - 1] else {
+                    continue
+                }
+                twoSum = threeSum - nums[j]
+                
+                left = j + 1
+                right = nums.count - 1
+                while left < right {
+                    if nums[left] + nums[right] == twoSum {
+                        res.append([nums[i], nums[j], nums[left], nums[right]])
+                        repeat {
+                            left += 1
+                        } while left < right && nums[left] == nums[left - 1]
+                            repeat {
+                                right -= 1
+                        } while left < right && nums[right] == nums[right + 1]
+                    } else if nums[left] + nums[right] < twoSum {
+                        repeat {
+                            left += 1
+                        } while left < right && nums[left] == nums[left - 1]
+                    } else {
+                        repeat {
+                            right -= 1
+                        } while left < right && nums[right] == nums[right + 1]
+                    }
+                }
+            }
+        }
+                
+        return res
+    }
+}

Array/SummaryRanges.swift

@@ -0,0 +1,31 @@
+/**
+ * Question Link: https://leetcode.com/problems/summary-ranges/
+ * Primary idea: Traverse the array and build string when num[i] != num[i - 1] + 1, 
+ *               note to handle the edge case when it goes to the end of the array
+ *
+ * Time Complexity: O(n), Space Complexity: O(n)
+ */
+
+class SummaryRanges {
+    func summaryRanges(nums: [Int]) -> [String] {
+        var res = [String]()
+        var str = ""
+        var start = 0
+        
+        guard nums.count > 0 else {
+            return res
+        }
+  
+        for i in 0 ... nums.count {
+            if i == nums.count || (i > 0 && nums[i] != nums[i - 1] + 1) {
+                str = "\(nums[start])"
+                if i - 1 != start {
+                    str += "->\(nums[i - 1])"
+                }
+                res.append(str)
+                start = i
+            }
+        }
+        return res
+    }
+}

Array/ThreeSum.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/3sum/
  * Primary idea: Sort the array, and traverse it, increment left or decrease right 
  *               predicated on their sum is greater or not than the target
- * Time Complexity: O(n^2), Space Complexity: O(n)
+ * Time Complexity: O(n^2), Space Complexity: O(nC3)
  */
 
 class ThreeSum {