diff --git a/20201221/main.go b/20201221/main.go
new file mode 100644
index 0000000..8aa43db
--- /dev/null
+++ b/20201221/main.go
@@ -0,0 +1,25 @@
+package main
+
+func main() {
+
+}
+
+func isMountain(nums []int) bool {
+	hasUp := false
+	hasDown := false
+	c := nums[0]
+	for i := 0; i < len(nums); i++ {
+		n := nums[i]
+		if !hasUp && !hasDown && n > c {
+			hasUp = true
+		}
+		if c > n {
+			hasDown = true
+		}
+		if n > c && hasDown {
+			return false
+		}
+		c = n
+	}
+	return hasDown && hasUp
+}
diff --git a/20201221/main_test.go b/20201221/main_test.go
new file mode 100644
index 0000000..da8f041
--- /dev/null
+++ b/20201221/main_test.go
@@ -0,0 +1,37 @@
+package main
+
+import "testing"
+
+func Test_isMountain(t *testing.T) {
+	type args struct {
+		nums []int
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			"test1",
+			args{[]int{2, 1}},
+			false,
+		},
+		{
+			"test2",
+			args{[]int{3, 5, 5}},
+			false,
+		},
+		{
+			"test3",
+			args{[]int{0, 3, 2, 1}},
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isMountain(tt.args.nums); got != tt.want {
+				t.Errorf("isMountain() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}