[Array] Add solution to Majority Element

Array/MajorityElement.swift

@@ -0,0 +1,29 @@
+/**
+ * Question Link: https://leetcode.com/problems/majority-element/
+ * Primary idea: traverse the array and track the majority element accordingly
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class MajorityElement {
+    func majorityElement(_ nums: [Int]) -> Int {
+        var major = nums.first!
+        var sum = 0
+
+        for num in nums {
+            if num == major {
+                sum += 1
+            } else {
+                sum -= 1
+            }
+
+            if sum == 0 {
+                major = num
+                sum = 1
+            }
+        }
+
+        return major
+    }
+}