diff --git a/problems/add-binary/add_binary.go b/problems/add-binary/add_binary.go
index 618b14970..1e5af18b8 100644
--- a/problems/add-binary/add_binary.go
+++ b/problems/add-binary/add_binary.go
@@ -1 +1,22 @@
 package add_binary
+
+func addBinary(a string, b string) string {
+	i, j := len(a)-1, len(b)-1
+	var carry byte = '0'
+	var bs []byte
+	for i >= 0 || j >= 0 || carry != '0' {
+		v := carry
+		if i >= 0 {
+			v += a[i] - '0'
+			i--
+		}
+		if j >= 0 {
+			v += b[j] - '0'
+			j--
+		}
+		carry = '0' + (v-'0')/2
+		v = '0' + (v-'0')%2
+		bs = append([]byte{v}, bs...)
+	}
+	return string(bs)
+}
diff --git a/problems/add-binary/add_binary_test.go b/problems/add-binary/add_binary_test.go
index 618b14970..11adb7846 100644
--- a/problems/add-binary/add_binary_test.go
+++ b/problems/add-binary/add_binary_test.go
@@ -1 +1,35 @@
 package add_binary
+
+import "testing"
+
+type caseType struct {
+	a        string
+	b        string
+	expected string
+}
+
+func TestAddBinary(t *testing.T) {
+	tests := [...]caseType{
+		{
+			a:        "11",
+			b:        "1",
+			expected: "100",
+		},
+		{
+			a:        "1010",
+			b:        "1011",
+			expected: "10101",
+		},
+		{
+			a:        "111",
+			b:        "111",
+			expected: "1110",
+		},
+	}
+	for _, tc := range tests {
+		output := addBinary(tc.a, tc.b)
+		if output != tc.expected {
+			t.Fatalf("input: %v %v, output: %v, expected: %v", tc.a, tc.b, output, tc.expected)
+		}
+	}
+}