diff --git a/20201209/main.go b/20201209/main.go
new file mode 100644
index 0000000..bbf97df
--- /dev/null
+++ b/20201209/main.go
@@ -0,0 +1,25 @@
+package main
+
+func main() {
+
+}
+
+func transform(src [][]int) [][]int {
+	for i := 0; i < len(src); i++ {
+		for j := i; j < len(src); j++ {
+			src[i][j], src[j][i] = src[j][i], src[i][j]
+		}
+	}
+	for i, l := range src {
+		src[i] = reverse(l)
+	}
+	return src
+}
+
+func reverse(src []int) []int {
+	l := len(src)
+	for i := 0; i < l/2; i++ {
+		src[i], src[l-i-1] = src[l-i-1], src[i]
+	}
+	return src
+}
diff --git a/20201209/main_test.go b/20201209/main_test.go
new file mode 100644
index 0000000..40c5a08
--- /dev/null
+++ b/20201209/main_test.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test_transform(t *testing.T) {
+	type args struct {
+		src [][]int
+	}
+	tests := []struct {
+		name string
+		args args
+		want [][]int
+	}{
+		{
+			"test1",
+			args{[][]int{
+				{1, 2, 3},
+				{4, 5, 6},
+				{7, 8, 9},
+			}},
+			[][]int{
+				{7, 4, 1},
+				{8, 5, 2},
+				{9, 6, 3},
+			},
+		},
+		{
+			"test2",
+			args{[][]int{
+				{5, 1, 9, 11},
+				{2, 4, 8, 10},
+				{13, 3, 6, 7},
+				{15, 14, 12, 16},
+			}},
+			[][]int{
+				{15, 13, 2, 5},
+				{14, 3, 4, 1},
+				{12, 6, 8, 9},
+				{16, 7, 10, 11},
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := transform(tt.args.src); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("transform() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_reverse(t *testing.T) {
+	type args struct {
+		src []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want []int
+	}{
+		{
+			"test1",
+			args{
+				[]int{1, 2, 3},
+			},
+			[]int{3, 2, 1},
+		},
+		{
+			"test2",
+			args{
+				[]int{1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3},
+			},
+			[]int{3, 2, 1, 3, 2, 1, 3, 2, 1, 3, 2, 1},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := reverse(tt.args.src); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("reverse() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}