[Array] add solution to Longest Consecutive Sequence

Array/LongestConsecutiveSequence.swift

@@ -0,0 +1,50 @@
+/**
+ * Question Link: https://leetcode.com/problems/longest-consecutive-sequence/
+ * Primary idea: Iterate the array and check all neighbor numbers with the help of two sets
+ *
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ */
+
+class LongestConsecutiveSequence {
+    func longestConsecutive(nums: [Int]) -> Int {
+        guard nums.count > 0 else {
+            return 0
+        }
+
+        var setNums = Set<Int>(nums)
+        var setDups = Set<Int>()
+        var longest = 1
+
+        for num in nums {
+            guard !setDups.contains(num) else {
+                continue
+            }
+
+            setDups.insert(num)
+            var len = 1
+
+            len += _helper(&setDups, &setNums, num, 1);
+            len += _helper(&setDups, &setNums, num, -1);
+
+            if (len > longest) {
+                longest = len;
+            }
+        }
+
+        return longest
+    }
+
+    private func _helper(inout setDups: Set<Int>, inout _ setNums: Set<Int>, _ num: Int, _ step: Int) -> Int {
+        var len = 0
+        var num = num
+
+        while setNums.contains(num + step) {
+            num += step
+            setDups.insert(num)
+            len += 1
+        }
+
+        return len
+    }
+}