Add vanilla merge sort example

inputs/passing/mergeSort.ts

@@ -0,0 +1,51 @@
+export default function main() {
+  let x = [7, 18, 9, 11, 16, 3, 8, 2, 5, 4, 6, 14, 15, 17, 10, 12, 1, 13];
+
+  return mergeSort(x, (a, b) => a - b);
+}
+
+function mergeSort<T>(vals: T[], cmp: (a: T, b: T) => number): T[] {
+  const len = vals.length;
+
+  if (len <= 1) {
+    return vals;
+  }
+
+  if (len === 2) {
+    if (cmp(vals[0], vals[1]) > 0) {
+      return [vals[1], vals[0]];
+    }
+
+    return vals;
+  }
+
+  const mid = vals.length / 2;
+
+  const leftSorted = mergeSort(vals.slice(0, mid), cmp);
+  const rightSorted = mergeSort(vals.slice(mid), cmp);
+
+  let res: T[] = [];
+
+  let left = 0;
+  const leftLen = leftSorted.length;
+  let right = 0;
+  const rightLen = rightSorted.length;
+
+  while (left < leftLen && right < rightLen) {
+    if (cmp(leftSorted[left], rightSorted[right]) <= 0) {
+      res.push(leftSorted[left++]);
+    } else {
+      res.push(rightSorted[right++]);
+    }
+  }
+
+  while (left < leftLen) {
+    res.push(leftSorted[left++]);
+  }
+
+  while (right < rightLen) {
+    res.push(rightSorted[right++]);
+  }
+
+  return res;
+}