Add problem 4 of chapter 8

src/chapter8/problem4.go

@@ -0,0 +1,17 @@
+package main
+
+// PowerSet returns all subset of give set
+func PowerSet(set []int) [][]int {
+	if len(set) == 0 {
+		return [][]int{
+			[]int{},
+		}
+	}
+
+	var res [][]int
+	for _, subset := range PowerSet(set[1:]) {
+		res = append(res, subset, append(subset, set[0]))
+	}
+
+	return res
+}

src/chapter8/problem4_test.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestPowerSet(t *testing.T) {
+	tests := map[string]struct {
+		in  []int
+		out [][]int
+	}{
+
+		"Should return empty subset when input is empty": {
+			in: []int{},
+			out: [][]int{
+				[]int{},
+			},
+		},
+
+		"Should return 2 subsets when input's length is 1": {
+			in: []int{1},
+			out: [][]int{
+				[]int{},
+				[]int{1},
+			},
+		},
+
+		"Should return 4 subsets when input's length is 2": {
+			in: []int{1, 2},
+			out: [][]int{
+				[]int{},
+				[]int{1},
+				[]int{2},
+				[]int{2, 1},
+			},
+		},
+
+		"Should return 8 subsets when input's length is 3": {
+			in: []int{1, 2, 3},
+			out: [][]int{
+				[]int{},
+				[]int{1},
+				[]int{2},
+				[]int{2, 1},
+				[]int{3},
+				[]int{3, 1},
+				[]int{3, 2},
+				[]int{3, 2, 1},
+			},
+		},
+
+		"Should return 16 subsets when input's length is 4": {
+			in: []int{1, 2, 3, 4},
+			out: [][]int{
+				[]int{},
+				[]int{1},
+				[]int{2},
+				[]int{2, 1},
+				[]int{3},
+				[]int{3, 1},
+				[]int{3, 2},
+				[]int{3, 2, 1},
+				[]int{4},
+				[]int{4, 1},
+				[]int{4, 2},
+				[]int{4, 2, 1},
+				[]int{4, 3},
+				[]int{4, 3, 1},
+				[]int{4, 3, 2},
+				[]int{4, 3, 2, 1},
+			},
+		},
+	}
+
+	for k, test := range tests {
+		t.Run(k, func(t *testing.T) {
+			out := PowerSet(test.in)
+			if !reflect.DeepEqual(out, test.out) {
+				t.Errorf("actual=%#v expected=%#v", out, test.out)
+			}
+		})
+	}
+}