feat: add merge sort (#38)

* feat: add merge sort

* chore: Fix typo

src/algorithms/merge-sort.js

@@ -0,0 +1,55 @@
+/*
+ * CODE: http://www.stoimen.com/blog/2010/07/02/friday-algorithms-javascript-merge-sort/
+ * 
+ * Mergesort is an efficient, general-purpose, comparison-based sorting 
+ algorithm. Most implementations produce a stable sort, which means that the 
+ implementation preserves the input order of equal elements in the sorted 
+ output. Mergesort is a divide and conquer algorithm that was invented by John 
+ von Neumann in 1945.
+ * 
+ * MORE INFO: https://en.wikipedia.org/wiki/Merge_sort
+ */
+function merge(arr, left, right) {
+  let buffer = [];
+
+  while (left.length && right.length) {
+    if (left[0] <= right[0]) {
+      buffer.push(left.shift());
+    } else {
+      buffer.push(right.shift());
+    }
+  }
+
+  while (left.length) {
+    buffer.push(left.shift());
+  }
+  while (right.length) {
+    buffer.push(right.shift());
+  }
+
+  // NOTE: This is NOT required for the sorting algorithm
+  trackMutations(arr, buffer);
+
+  return buffer;
+}
+
+/*
+ * Note: this function would normally not be present
+ * It is necessary to visualize the "buffer" required by merge sort
+ */
+function trackMutations(arr, updated) {
+  for (let i = 0; i < updated.length; i++) {
+    arr[i] = updated[i];
+  }
+}
+
+export function mergeSort(arr, sorted = arr, len = sorted.length) {
+  if (sorted.length <= 1) {
+    return sorted;
+  }
+  const middle = parseInt(len / 2);
+  let left = sorted.slice(0, middle);
+  let right = sorted.slice(middle, len);
+
+  return merge(arr, mergeSort(arr, left), mergeSort(arr, right));
+}