diff --git a/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution.go b/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution.go
new file mode 100644
index 0000000..b3104da
--- /dev/null
+++ b/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution.go	
@@ -0,0 +1,41 @@
+package Challenge3_Count_of_Structurally_Unique_Binary_Search_Trees
+
+/*
+已经在challenge2中实现
+ref: https://leetcode-cn.com/problems/unique-binary-search-trees/
+*/
+func numTrees(n int) int {
+	var bfs func(int, int) int
+	var m = make(map[Range]int)
+	bfs = func(start, end int) int {
+		var res = 0
+		if start == end {
+			return 1
+		}
+		if v, ok := m[Range{start, end}]; ok {
+			return v
+		}
+		for i := start; i <= end; i++ {
+			if i == start {
+				res += bfs(i+1, end)
+				continue
+			}
+			if i == end {
+				res += bfs(start, i-1)
+				continue
+			}
+			left := bfs(start, i-1)
+			right := bfs(i+1, end)
+			res += left * right
+		}
+		m[Range{start, end}] = res
+		return res
+	}
+
+	return bfs(1, n)
+}
+
+type Range struct {
+	start int
+	end   int
+}
diff --git a/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution_test.go b/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution_test.go
new file mode 100644
index 0000000..1be7a6e
--- /dev/null
+++ b/Pattern10 - Subsets/Challenge3-Count_of_Structurally_Unique_Binary_Search_Trees/solution_test.go	
@@ -0,0 +1,26 @@
+package Challenge3_Count_of_Structurally_Unique_Binary_Search_Trees
+
+import "testing"
+
+func Test_NumTrees(t *testing.T){
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{"case1", args{2}, 2},
+		{"case2", args{3}, 5},
+		{"case3", args{1}, 1},
+		{"case4", args{4}, 14},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := numTrees(tt.args.n); got != tt.want {
+				t.Errorf("numTrees() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
\ No newline at end of file