feat: add geometric sequence triplets solution

Author: ineBallardin

go/Hash Maps and Sets/geometric_sequence_triplets.go

@@ -1 +1,27 @@
 package hashmapsandsets
+
+func GeometricSequenceTriplets(nums []int, r int) int {
+	// Use 'map' to ensure the default value of 0 is returned when
+	// accessing a key that doesn’t exist in the hash map. This effectively sets
+	// the default frequency of all elements to 0.
+	leftMap := make(map[int]int)
+	rightMap := make(map[int]int)
+	count := 0
+	// Populate 'rightMap' with the frequency of each element in the array.
+	for _, x := range nums {
+		rightMap[x]++
+	}
+	// Search for geometric triplets that have x as the center.
+	for _, x := range nums {
+		// Decrement the frequency of x in 'rightMap' since x is now being
+		// processed and is no longer to the right.
+		rightMap[x]--
+		if x%r == 0 {
+			count += leftMap[x/r] * rightMap[x*r]
+		}
+		// Increment the frequency of x in 'leftMap' since it'll be a part of the
+		// left side of the array once we iterate to the next value of x.
+		leftMap[x]++
+	}
+	return count
+}