cpp-course/modules/03_functions/src/recursion.cpp

#include "recursion.hpp"

namespace functions {

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