add solution for count-triplets

readme.md

@@ -1,4 +1,4 @@
-## Algorithms (19/ 167)
+## Algorithms (20/ 167)
 
 | Name                                                      | Tags                    | Solution                                                                                                                            |
 | --------------------------------------------------------- | ----------------------- | ----------------------------------------------------------------------------------------------------------------------------------- |
@@ -11,7 +11,7 @@
 | Ransom Note                                               | `Hash Tables`           | [TypeScript](./src/algorithms/hashtables/ransom-note)                                                                               |
 | Two Strings                                               | `Hash Tables`           | [TypeScript](./src/algorithms/hashtables/two-strings)                                                                               |
 | Sherlock and Anagrams                                     | `Hash Tables`           | [TypeScript](./src/algorithms/hashtables/sherlock-and-anagrams)                                                                     |
-| Count Triplets                                            | `Hash Tables`           |                                                                                                                                     |
+| Count Triplets                                            | `Hash Tables`           | [TypeScript](./src/algorithms/hashtables/count-triplets)                                                                            |
 | Frequency Queries                                         | `Hash Tables`           |                                                                                                                                     |
 | Making Anagrams                                           | `Strings`               | [TypeScript](./src/algorithms/strings/make-anagram)                                                                                 |
 | Alternating Characters                                    | `Strings`               |                                                                                                                                     |

src/algorithms/hashtables/count-triplets/count-triplets.spec.ts

@@ -0,0 +1,14 @@
+import { countTriplets } from './count-triplets'
+
+describe('Count triplets', () => {
+  it('should return the number of triplets', () => {
+    expect(countTriplets([1, 2, 2, 4], 2)).toBe(2)
+    expect(countTriplets([1, 3, 9, 9, 27, 81], 3)).toBe(6)
+    expect(countTriplets([1, 5, 5, 25, 125], 5)).toBe(4)
+    expect(countTriplets([1, 1, 1, 1, 1], 1)).toBe(10)
+  })
+
+  it('should return 0 if there are no triplets', () => {
+    expect(countTriplets([1, 2, 3], 1)).toBe(0)
+  })
+})

src/algorithms/hashtables/count-triplets/count-triplets.ts

@@ -0,0 +1,34 @@
+/**
+ * Count Triplets
+ *
+ * Given an array, find the number of triplets of incides (i, j, k) such that the elements at those indeces are in geometric progression
+ * for a given common ratio r and i < j < k
+ *
+ * arr = [1, 4, 16, 64] r = 4
+ *
+ * [1, 4, 16] and [4, 16, 64] at indices (0, 1, 2) and (1, 2, 3) return 2
+ *
+ *
+ * */
+export function countTriplets(arr: number[], r: number): number {
+  let count = 0
+  const map = new Map<number, number>()
+  const pairs = new Map<number, number>()
+
+  for (let i = 0; i < arr.length; i++) {
+    const num = arr[i]
+
+    if (map.has(num)) {
+      count += map.get(num) || 0
+    }
+
+    if (pairs.has(num)) {
+      const pair = pairs.get(num) || 0
+      map.set(num * r, (map.get(num * r) || 0) + pair)
+    }
+
+    pairs.set(num * r, (pairs.get(num * r) || 0) + 1)
+  }
+
+  return count
+}