feat(core): add quick select

packages/core/src/index.ts

@@ -13,6 +13,7 @@ export * from './bubbleSort';
 export * from './insertionSort';
 export * from './linearSearch';
 export * from './mergeSort';
+export * from './quickSelect';
 export * from './quickSort';
 export * from './radixSort';
 export * from './selectionSort';

packages/core/src/quickSelect/index.ts

@@ -0,0 +1 @@
+export { kthLargest, kthSmallest } from './quickSelect';

packages/core/src/quickSelect/quickSelect.test.ts

@@ -0,0 +1,13 @@
+import { kthLargest, kthSmallest } from './quickSelect';
+
+describe('quickSelect', () => {
+  it('returns the kth largest element', () => {
+    const arr = [3, 2, 1, 5, 6, 4];
+    expect(kthLargest(arr, 2)).toBe(5);
+  });
+
+  it('returns the kth smallest element with duplicate inputs', () => {
+    const arr = [-1, -1, 0, 5, 12];
+    expect(kthSmallest(arr, 3)).toEqual(0);
+  });
+});

packages/core/src/quickSelect/quickSelect.ts

@@ -0,0 +1,31 @@
+function quickSelect(arr: number[], k: number, left = 0, right = arr.length - 1): number {
+  const pivot = arr[right];
+  let p = left;
+
+  for (let i = left; i < right; i++) {
+    if (arr[i] <= pivot) {
+      [arr[i], arr[p]] = [arr[p], arr[i]];
+      p++;
+    }
+  }
+
+  [arr[p], arr[right]] = [arr[right], arr[p]];
+
+  if (p === k) {
+    return arr[p];
+  }
+
+  if (p > k) {
+    return quickSelect(arr, k, left, p - 1);
+  } else {
+    return quickSelect(arr, k, p + 1, right);
+  }
+}
+
+export function kthLargest(arr: number[], k: number): number {
+  return quickSelect(arr, arr.length - k);
+}
+
+export function kthSmallest(arr: number[], k: number): number {
+  return quickSelect(arr, k - 1);
+}