add Join func

mr.go

@@ -1,5 +1,7 @@
 package mr
 
+import "strings"
+
 func Map[T any, R any](ts []T, f func(T) R) []R {
 	nt := make([]R, 0, len(ts))
 	for _, t := range ts {
@@ -107,3 +109,14 @@ func Paginate[T any](list []T, page, size int) []T {
 	}
 	return list[offset : offset+size]
 }
+
+func Join[T any](list []T, sep string, f func(T) string) string {
+	var builder strings.Builder
+	for i := 0; i < len(list); i++ {
+		if i > 0 {
+			builder.WriteString(sep)
+		}
+		builder.WriteString(f(list[i]))
+	}
+	return builder.String()
+}

mr_test.go

@@ -1,6 +1,7 @@
 package mr
 
 import (
+	"strconv"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -263,3 +264,12 @@ func TestPaginate(t *testing.T) {
 	result = Paginate(ts, 2, 2)
 	assert.Equal(t, expected, result)
 }
+
+func TestJoin(t *testing.T) {
+	str := Join(test_int_arr, ",", func(i int) string { return strconv.Itoa(i) })
+	assert.Equal(t, "1,2,3,4,5", str)
+
+	ts := []int{}
+	str = Join(ts, ",", func(i int) string { return strconv.Itoa(i) })
+	assert.Empty(t, str)
+}