From e6bdb3bdd84deddf09678a3ce88f19fe1c90aaf4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?David=20Gil=20de=20G=C3=B3mez=20P=C3=A9rez?=
 <david.gil.de.gomez@columbiaroad.com>
Date: Wed, 12 Aug 2026 17:30:55 +0300
Subject: [PATCH] Solved: 12 - Recursion

---
 modules/03_functions/src/recursion.cpp | 33 +++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/modules/03_functions/src/recursion.cpp b/modules/03_functions/src/recursion.cpp
index 8e3bee4..052db9a 100644
--- a/modules/03_functions/src/recursion.cpp
+++ b/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