Solved: 12 - Recursion

modules/03_functions/src/recursion.cpp

@@ -2,8 +2,35 @@
 
 namespace functions {
 
-auto fibonacci(int n) -> long long { (void)n; return 0; }
-auto sum_digits(int n) -> int { (void)n; return 0; }
-auto is_palindrome(int n) -> bool { (void)n; return false; }
+auto fibonacci(const int n) -> long long {
+    if (n == 0) return 0;
+    if (n == 1) return 1;
+    return fibonacci(n - 1) + fibonacci(n - 2);
+}
+
+auto sum_digits(const int n) -> int {
+    if (n < 10) return n;
+    return n % 10 + sum_digits(n / 10);
+}
+
+// Advanced recursion with recursive lambda helper
+auto is_palindrome(const int n) -> bool {
+    auto f = [](const auto& self, const int x, const int d) -> bool {
+        if (x < 0) return false;
+        if (x < 10) return true;
+        if (x / d != x % 10) return false;
+        if (d < 10) return true;
+        const int m = (x / d) / 10;
+        return self(self, m, d / 100);
+    };
+    if (n < 0) return false;
+    if (n < 10) return true;
+    int d = 1;
+    while (n / d >= 10) {
+        d *= 10;
+    }
+    return f(f, n, d);
+
+}
 
 }  // namespace functions