Merge pull request TheAlgorithms#29 from Polyneue/enhancement/TheAlgorithms#14-add-merge-sort

TheAlgorithms#14 - Adding merge sort algorithm

Author: ms10398

Sorts/mergeSort.js

@@ -0,0 +1,49 @@
+/**
+ * Merge Sort is an algorithm where the main list is divided down into two half
+ * sized lists, which then have merge sort called on these two smaller lists 
+ * recursively until there is only a sorted list of one.
+ * 
+ * On the way up the recursive calls, the lists will be merged together inserting
+ * the smaller value first, creating a larger sorted list.
+ */
+
+/**
+ * Sort and merge two given arrays
+ * @param {Array} list1 - sublist to break down
+ * @param {Array} list2 - sublist to break down
+ * @return {Array} merged list
+ */
+function merge(list1, list2) {
+  var results = [];
+
+  while(list1.length && list2.length) {
+    if (list1[0] <= list2[0]) {
+      results.push(list1.shift());
+    } else {
+      results.push(list2.shift());
+    }
+  }
+  return results.concat(list1, list2);
+}
+
+/**
+ * Break down the lists into smaller pieces to be merged
+ * @param {Array} list - list to be sorted
+ * @return {Array} sorted list
+ */
+function mergeSort(list) {
+  if (list.length < 2) return list;
+
+  var listHalf = Math.floor(list.length/2);
+  var subList1 = list.slice(0, listHalf);
+  var subList2 = list.slice(listHalf, list.length);
+
+  return merge(mergeSort(subList1), mergeSort(subList2));
+}
+
+// Merge Sort Example
+var unsortedArray = [10, 5, 3, 8, 2, 6, 4, 7, 9, 1];
+var sortedArray = mergeSort(unsortedArray);
+
+console.log('Before:', unsortedArray, 'After:', sortedArray);
+