day 2 solution. both with division and without

day2/problem.go

@@ -0,0 +1,35 @@
+package day2
+
+// ProductsExceptI returns an array with the products of all numbers
+// except the value at the i-th position.
+// O(N) time complexity.
+// O(1) space complexity.
+func ProductsExceptI(arr []int) []int {
+	result := make([]int, len(arr))
+	total := 1
+	for _, v := range arr {
+		total *= v
+	}
+	for i, v := range arr {
+		result[i] = total / v
+	}
+	return result
+}
+
+// ProductsExceptINoDivision returns an array with the products of all numbers
+// except the value at the i-th position. Without using division.
+// O(N^2) time complexity.
+// O(1) space complexity.
+func ProductsExceptINoDivision(arr []int) []int {
+	result := make([]int, len(arr))
+	for i := range arr {
+		total := 1
+		for j, w := range arr {
+			if i != j {
+				total *= w
+			}
+		}
+		result[i] = total
+	}
+	return result
+}

day2/problem_test.go

@@ -0,0 +1,49 @@
+package day2
+
+import (
+	"reflect"
+	"testing"
+)
+
+var testcases = []struct {
+	input, expected []int
+}{
+	{[]int{1, 2, 3, 4, 5}, []int{120, 60, 40, 30, 24}},
+	{[]int{3, 2, 1}, []int{2, 3, 6}},
+}
+
+func TestProductsExceptI(t *testing.T) {
+	t.Run("ProductsExceptI", func(t *testing.T) {
+		t.Parallel()
+		for _, tc := range testcases {
+			if result := ProductsExceptI(tc.input); !reflect.DeepEqual(result, tc.expected) {
+				t.Error("didn't get what I expected")
+			}
+		}
+	})
+	t.Run("ProductsExceptINoDivision", func(t *testing.T) {
+		t.Parallel()
+		for _, tc := range testcases {
+			if result := ProductsExceptINoDivision(tc.input); !reflect.DeepEqual(result, tc.expected) {
+				t.Error("didn't get what I expected")
+			}
+		}
+	})
+}
+
+func BenchmarkProductsExceptI(b *testing.B) {
+	b.Run("ProductsExceptI", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			for _, tc := range testcases {
+				ProductsExceptI(tc.input)
+			}
+		}
+	})
+	b.Run("ProductsExceptINoDivision", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			for _, tc := range testcases {
+				ProductsExceptI(tc.input)
+			}
+		}
+	})
+}