[Array] Add a solution to Majority Element II

Author: Yi Gu

Array/MajorityElementII.swift

@@ -0,0 +1,56 @@
+/**
+ * Question Link: https://leetcode.com/problems/majority-element-ii/
+ * Primary idea: traverse the array and track the majority element accordingly, do not
+ *				 forget to verify they are valid after first iteration
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+ class MajorityElementII {
+    func majorityElement(_ nums: [Int]) -> [Int] {
+        var num0: Int?
+        var num1: Int?
+        var count0 = 0
+        var count1 = 0
+        var res = [Int]()
+        
+        for num in nums {
+            if let num0 = num0, num0 == num {
+                count0 += 1
+            } else if let num1 = num1, num1 == num {
+                count1 += 1
+            } else if count0 == 0 {
+                num0 = num
+                count0 = 1
+            } else if count1 == 0 {
+                num1 = num
+                count1 = 1
+            } else {
+                count0 -= 1
+                count1 -= 1
+            }
+        }
+        
+        count0 = 0
+        count1 = 0
+        
+        for num in nums {
+            if num == num0 {
+                count0 += 1
+            }
+            if num == num1 {
+                count1 += 1
+            }
+        }
+        
+        if count0 > nums.count / 3 {
+            res.append(num0!)
+        }
+        if count1 > nums.count / 3 {
+            res.append(num1!)
+        }
+        
+        return res
+    }
+}