diff --git a/problems/add-binary/add_binary.go b/problems/add-binary/add_binary.go
index 1e5af18b8..283302d79 100644
--- a/problems/add-binary/add_binary.go
+++ b/problems/add-binary/add_binary.go
@@ -1,22 +1,20 @@
-package add_binary
+package problem_67
 
 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' {
+	ans, l1, l2, carry := "", len(a)-1, len(b)-1, byte('0')
+	for l1 >= 0 || l2 >= 0 || carry != '0' {
 		v := carry
-		if i >= 0 {
-			v += a[i] - '0'
-			i--
+		if l1 >= 0 {
+			v += a[l1] - '0'
+			l1--
 		}
-		if j >= 0 {
-			v += b[j] - '0'
-			j--
+		if l2 >= 0 {
+			v += b[l2] - '0'
+			l2--
 		}
 		carry = '0' + (v-'0')/2
 		v = '0' + (v-'0')%2
-		bs = append([]byte{v}, bs...)
+		ans = string(v) + ans
 	}
-	return string(bs)
+	return ans
 }
diff --git a/problems/add-binary/add_binary_test.go b/problems/add-binary/add_binary_test.go
index 11adb7846..071a57d26 100644
--- a/problems/add-binary/add_binary_test.go
+++ b/problems/add-binary/add_binary_test.go
@@ -1,4 +1,4 @@
-package add_binary
+package problem_67
 
 import "testing"
 
diff --git a/problems/add-strings/add_strings.go b/problems/add-strings/add_strings.go
index 318fa6e85..a50f25582 100644
--- a/problems/add-strings/add_strings.go
+++ b/problems/add-strings/add_strings.go
@@ -1 +1,20 @@
-package add_strings
+package problem_415
+
+func addStrings(num1 string, num2 string) string {
+	ans, l1, l2, carry := "", len(num1)-1, len(num2)-1, byte('0')
+	for l1 >= 0 || l2 >= 0 || carry != '0' {
+		v := carry
+		if l1 >= 0 {
+			v += num1[l1] - '0'
+			l1--
+		}
+		if l2 >= 0 {
+			v += num2[l2] - '0'
+			l2--
+		}
+		carry = '0' + (v-'0')/10
+		v = '0' + (v-'0')%10
+		ans = string(v) + ans
+	}
+	return ans
+}
diff --git a/problems/add-strings/add_strings_test.go b/problems/add-strings/add_strings_test.go
index 318fa6e85..986cb15c1 100644
--- a/problems/add-strings/add_strings_test.go
+++ b/problems/add-strings/add_strings_test.go
@@ -1 +1,40 @@
-package add_strings
+package problem_415
+
+import "testing"
+
+type caseType struct {
+	num1     string
+	num2     string
+	expected string
+}
+
+func TestAddStrings(t *testing.T) {
+	tests := [...]caseType{
+		{
+			num1:     "0",
+			num2:     "0",
+			expected: "0",
+		},
+		{
+			num1:     "1",
+			num2:     "2",
+			expected: "3",
+		},
+		{
+			num1:     "9",
+			num2:     "9",
+			expected: "18",
+		},
+		{
+			num1:     "100",
+			num2:     "999",
+			expected: "1099",
+		},
+	}
+	for _, tc := range tests {
+		output := addStrings(tc.num1, tc.num2)
+		if output != tc.expected {
+			t.Fatalf("input: %v, output: %v, expected: %v", tc, output, tc.expected)
+		}
+	}
+}