[Sort] add Solution to Wiggle Sort I, II

Author: Yi Gu

Sort/WiggleSort.swift

@@ -0,0 +1,45 @@
+/**
+ * Question Link: https://leetcode.com/problems/wiggle-sort/
+ * Primary idea: Iterate the array and swap the largest one to the middle
+ * Time Complexity: O(n), Space Complexity: O(1)
+ */
+
+class WiggleSort {
+    func wiggleSort(inout nums: [Int]) {
+        guard nums.count >= 2 else {
+            return
+        }
+        
+        for i in 1.stride(to: nums.count, by: 2) {
+            let largestIndex = _getLargest(&nums, i - 1, i, i + 1)
+            if i != largestIndex {
+                _swap(&nums, largestIndex, i)
+            }
+        }
+    }
+    
+    private func _swap(inout nums: [Int], _ p: Int, _ q: Int) {
+        let temp = nums[p]
+        nums[p] = nums[q]
+        nums[q] = temp
+    }
+    
+    private func _getLargest(inout nums: [Int], _ x: Int, _ y: Int, _ z: Int) -> Int {
+        let xVal = _isValid(x, nums.count) ? nums[x] : Int.min
+        let yVal = _isValid(y, nums.count) ? nums[y] : Int.min
+        let zVal = _isValid(z, nums.count) ? nums[z] : Int.min
+        let maxVal = max(xVal, yVal, zVal)
+        
+        if maxVal == xVal {
+            return x
+        } else if maxVal == yVal {
+            return y
+        } else {
+            return z
+        }
+    }
+    
+    private func _isValid(index: Int, _ len: Int) -> Bool {
+        return index >= 0 && index < len
+    }
+}

Sort/WiggleSortII.swift

@@ -0,0 +1,24 @@
+/**
+ * Question Link: https://leetcode.com/problems/wiggle-sort-ii/
+ * Primary idea: Sort and re-arrange the array 
+ * Time Complexity: O(nlogn), Space Complexity: O(n)
+ */
+
+class WiggleSortII {
+    func wiggleSort(inout nums: [Int]) {
+        let temp = nums.sort({$0 < $1})
+        
+        var m = temp.count
+        var n = (m + 1) / 2
+        
+        for i in 0 ..< nums.count {
+            if i & 1 == 0 {
+                n = n - 1
+                nums[i] = temp[n]
+            } else {
+                m = m - 1
+                nums[i] = temp[m]
+            }
+        }
+    }
+}