Add problem 7 of chapter 8

src/chapter8/problem7.go

@@ -0,0 +1,32 @@
+package chapter8
+
+// Permutations returns permutations of givien string
+func Permutations(str string) []string {
+	if len(str) == 0 {
+		return []string{""}
+	}
+
+	var res []string
+	for i := 0; i < len(str); i++ {
+		subs := Permutations(TrimAt(str, i))
+		for _, sub := range subs {
+			res = append(res, string(str[i])+sub)
+		}
+	}
+
+	return res
+}
+
+// TrimAt trims a char from given string by given index
+func TrimAt(str string, i int) string {
+	switch {
+	case i < 0 || len(str) == 0 || i >= len(str):
+		return str
+	case i == len(str)-1:
+		return str[:len(str)-1]
+	case i == 0:
+		return str[1:]
+	default:
+		return string(append([]byte(str[0:i]), str[i+1:]...))
+	}
+}

src/chapter8/problem7_test.go

@@ -0,0 +1,48 @@
+package chapter8
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestPermutations(t *testing.T) {
+	tests := map[string]struct {
+		in  string
+		out []string
+	}{
+		"should return slice of empty string when empty string is given": {
+			in:  "",
+			out: []string{""},
+		},
+		"should return 1 permutations when a is given": {
+			in:  "a",
+			out: []string{"a"},
+		},
+		"should return 2 permutations when ab is given": {
+			in:  "ab",
+			out: []string{"ab", "ba"},
+		},
+		"should return 6 permutations when abc is given": {
+			in:  "abc",
+			out: []string{"abc", "acb", "bac", "bca", "cab", "cba"},
+		},
+		"should return 24 permutations when abcd is given": {
+			in: "abcd",
+			out: []string{
+				"abcd", "abdc", "acbd", "acdb", "adbc", "adcb",
+				"bacd", "badc", "bcad", "bcda", "bdac", "bdca",
+				"cabd", "cadb", "cbad", "cbda", "cdab", "cdba",
+				"dabc", "dacb", "dbac", "dbca", "dcab", "dcba",
+			},
+		},
+	}
+
+	for k, test := range tests {
+		t.Run(k, func(t *testing.T) {
+			out := Permutations(test.in)
+			if !reflect.DeepEqual(out, test.out) {
+				t.Errorf("actual=%+v expected=%+v", out, test.out)
+			}
+		})
+	}
+}