Merge pull request #338 from soapyigu/Array

Add a solution to Subarrays with K Different Integers

Author: soapyigu

Array/SubarraysKDifferentIntegers.swift

@@ -0,0 +1,59 @@
+/**
+ * Question Link: https://leetcode.com/problems/subarrays-with-k-different-integers/
+ * Primary idea: Sliding window. 
+ * Assuming i1 < i2 < i3, (i1,j) and (i3,j) are valid, (i2,j) is definitely valid. 
+ * However, assuming i3 < i4, (i4,j) is is only invalid when the distance between i4 and j is less than k. 
+ * Using 2 sliding windows could solve the problem.
+ *
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ */
+
+class SubarraysKDifferentIntegers {
+    func subarraysWithKDistinct(_ nums: [Int], _ k: Int) -> Int {
+        var left1 = 0, left2 = 0, window1 = Window(), window2 = Window(), uniqueSubarrayCount = 0
+    
+        for right in 0..<nums.count {
+            let num = nums[right]
+            window1.add(num)
+            window2.add(num)
+
+            while window1.uniqueNum > k {
+                window1.remove(nums[left1])
+                left1 += 1
+            }
+
+            while window2.uniqueNum >= k {
+                window2.remove(nums[left2])
+                left2 += 1
+            }
+
+            uniqueSubarrayCount += (left2 - left1)
+        }
+
+        return uniqueSubarrayCount
+    }
+
+    class Window {
+        var uniqueNum = 0
+        private var numFreq = [Int: Int]()
+
+        func add(_ num: Int) {
+            numFreq[num, default: 0] += 1
+            if numFreq[num] == 1 {
+                uniqueNum += 1
+            }
+        }
+
+        func remove(_ num: Int) {
+            if let freq = numFreq[num] {
+                numFreq[num] = freq - 1
+
+                if freq == 1 {
+                    uniqueNum -= 1
+                }
+            }
+        }
+    }
+}
+