diff --git a/problems/number-of-digit-one/number_of_digit_one.go b/problems/number-of-digit-one/number_of_digit_one.go
index 800a3e308..c6edca3eb 100644
--- a/problems/number-of-digit-one/number_of_digit_one.go
+++ b/problems/number-of-digit-one/number_of_digit_one.go
@@ -1 +1,14 @@
-package number_of_digit_one
+package problem_233
+
+func countDigitOne(n int) int {
+	ans, b, x := 0, 1, 0
+	for b <= n {
+		x = n / b
+		ans += (x + 8) / 10 * b
+		if x%10 == 1 {
+			ans += n%b + 1
+		}
+		b *= 10
+	}
+	return ans
+}
diff --git a/problems/number-of-digit-one/number_of_digit_one_test.go b/problems/number-of-digit-one/number_of_digit_one_test.go
index 800a3e308..0b72bc0e4 100644
--- a/problems/number-of-digit-one/number_of_digit_one_test.go
+++ b/problems/number-of-digit-one/number_of_digit_one_test.go
@@ -1 +1,67 @@
-package number_of_digit_one
+package problem_233
+
+import "testing"
+
+type caseType struct {
+	input int
+	expected int
+}
+
+func TestCountDigitOne(t *testing.T) {
+	tests := [...]caseType{
+		{
+			input: 0,
+			expected: 0,
+		},
+		{
+			input: 1,
+			expected: 1,
+		},
+		{
+			input: 12,
+			expected: 5,
+		},
+		{
+			input: 99,
+			expected: 20,
+		},
+		{
+			input: 100,
+			expected: 21,
+		},
+		{
+			input: 123,
+			expected: 57,
+		},
+		{
+			input: 1234,
+			expected: 689,
+		},
+		{
+			input: 12345,
+			expected: 8121,
+		},
+		{
+			input: 123456,
+			expected: 93553,
+		},
+		{
+			input: 1234567,
+			expected: 1058985,
+		},
+		{
+			input: 12345678,
+			expected: 11824417,
+		},
+		{
+			input: 123456789,
+			expected: 130589849,
+		},
+	}
+	for _, tc := range tests {
+		output := countDigitOne(tc.input)
+		if output != tc.expected {
+			t.Fatalf("input: %v, output: %v, expected: %v", tc.input, output, tc.expected)
+		}
+	}
+}
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