Add heapsort

snippets/heapsort.md

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+---
+title: heapsort
+tags: algorithm,array,recursion,advanced
+---
+
+Sorts an array of numbers, using the heapsort algorithm.
+
+- Use recursion.
+- Use the spread operator (`...`) to clone the original array, `arr`.
+- Use closures to declare a variable, `l`, and a function `heapify`.
+- Use a `for` loop and `Math.floor()` in combination with `heapify` to create a max heap from the array.
+- Use a `for` loop to repeatedly narrow down the considered range, using `heapify` and swapping values as necessary in order to sort the cloned array.
+
+```js
+const heapsort = arr => {
+  const a = [...arr];
+  let l = a.length;
+
+  const heapify = (a, i) => {
+    const left = 2 * i + 1;
+    const right = 2 * i + 2;
+    let max = i;
+    if (left < l && a[left] > a[max]) max = left;
+    if (right < l && a[right] > a[max]) max = right;
+    if (max !== i) {
+      [a[max], a[i]] = [a[i], a[max]];
+      heapify(a, max);
+    }
+  };
+
+  for (let i = Math.floor(l / 2); i >= 0; i -= 1) heapify(a, i);
+  for (i = a.length - 1; i > 0; i--) {
+    [a[0], a[i]] = [a[i], a[0]];
+    l--;
+    heapify(a, 0);
+  }
+  return a;
+};
+```
+
+```js
+heapsort([6, 3, 4, 1]); // [1, 3, 4, 6]
+```