added mergesort.swift

Author: kelvinlauKL

Mergesort/Mergesort.swift

@@ -0,0 +1,56 @@
+//
+//  Mergesort.swift
+//  
+//
+//  Created by Kelvin Lau on 2016-02-03.
+//
+//
+
+func mergeSort(array: [Int]) -> [Int] {
+  guard array.count > 1 else { return array } // 1
+  let middleIndex = array.count / 2 // 2
+
+  let leftArray = mergeSort(Array(array[0..<middleIndex])) // 3
+
+  let rightArray = mergeSort(Array(array[middleIndex..<array.count])) // 4
+
+  return merge(leftPile: leftArray, rightPile: rightArray) // 5
+}
+
+func merge(leftPile leftPile: [Int], rightPile: [Int]) -> [Int] {
+  // 1
+  var leftIndex = 0
+  var rightIndex = 0
+
+  // 2
+  var orderedPile = [Int]()
+
+  // 3
+  while leftIndex < leftPile.count && rightIndex < rightPile.count {
+    if leftPile[leftIndex] < rightPile[rightIndex] {
+      orderedPile.append(leftPile[leftIndex])
+      leftIndex += 1
+    } else if leftPile[leftIndex] > rightPile[rightIndex] {
+      orderedPile.append(rightPile[rightIndex])
+      rightIndex += 1
+    } else {
+      orderedPile.append(leftPile[leftIndex])
+      leftIndex += 1
+      orderedPile.append(rightPile[rightIndex])
+      rightIndex += 1
+    }
+  }
+
+  // 4
+  while leftIndex < leftPile.count {
+    orderedPile.append(leftPile[leftIndex])
+    leftIndex += 1
+  }
+
+  while rightIndex < rightPile.count {
+    orderedPile.append(rightPile[rightIndex])
+    rightIndex += 1
+  }
+
+  return orderedPile
+}