diff --git a/.vscode/settings.json b/.vscode/settings.json
index 32704f3..c0bcd4c 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,3 +1,6 @@
 {
-    "C_Cpp.default.compilerPath": "/usr/bin/gcc"
+    "C_Cpp.default.compilerPath": "/usr/bin/gcc",
+    "files.associations": {
+        "sum_of_digits.h": "c"
+    }
 }
\ No newline at end of file
diff --git a/sum_of_digits.c b/sum_of_digits.c
index 29a8347..86af716 100644
--- a/sum_of_digits.c
+++ b/sum_of_digits.c
@@ -20,12 +20,21 @@ The sum of digits can be calculated as n mod 10 + sum of digits of n / 10 as lon
 
 */
 
-// TODO: the recursive implementation
 int sum_of_digits_recursive(int n) {
-    return 0;
+    if (n == 0) {
+        return 0;
+    }
+    
+    return n % 10 + sum_of_digits_recursive(n / 10);
 }
 
-// TODO: the iterative implementation
 int sum_of_digits_iterative(int n) {
-    return 0;
+    int sum = 0;
+    
+    while (n > 0) {
+        sum += n % 10;
+        n = n / 10;
+    }
+    
+    return sum;
 }
\ No newline at end of file