Add problem 9 of chapter 8

src/chapter8/problem9.go

@@ -0,0 +1,32 @@
+package main
+
+// Parens returns all valid parentheses by given number of pairs
+func Parens(num int) []string {
+	var (
+		res []string
+		bs  []byte
+	)
+
+	ParensR(num, 0, 0, bs, &res)
+
+	return res
+}
+
+// ParensR adds valid parentheses with recursion
+func ParensR(num, leftCnt, rightCnt int, bs []byte, res *[]string) {
+	if num < leftCnt || num < rightCnt {
+		return
+	}
+	if num == leftCnt && num == rightCnt {
+		*res = append(*res, string(bs))
+		return
+	}
+
+	if leftCnt < num {
+		ParensR(num, leftCnt+1, rightCnt, append(bs, '('), res)
+	}
+
+	if rightCnt < num && rightCnt < leftCnt {
+		ParensR(num, leftCnt, rightCnt+1, append(bs, ')'), res)
+	}
+}

src/chapter8/problem9_test.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestParens(t *testing.T) {
+	tests := map[string]struct {
+		in  int
+		out []string
+	}{
+		"Input is -1": {
+			in:  -1,
+			out: []string(nil),
+		},
+		"Input is 0": {
+			in: 0,
+			out: []string{
+				"",
+			},
+		},
+		"Input is 1": {
+			in: 1,
+			out: []string{
+				"()",
+			},
+		},
+		"Input is 2": {
+			in: 2,
+			out: []string{
+				"(())",
+				"()()",
+			},
+		},
+		"Input is 3": {
+			in: 3,
+			out: []string{
+				"((()))",
+				"(()())",
+				"(())()",
+				"()(())",
+				"()()()",
+			},
+		},
+	}
+
+	for k, test := range tests {
+		t.Run(k, func(t *testing.T) {
+			out := Parens(test.in)
+			if !reflect.DeepEqual(out, test.out) {
+				t.Errorf("actual=%+v expected=%+v", out, test.out)
+			}
+		})
+	}
+}