Fibonacci funtion added (#767)

* finbonacci funtion added

* finbonacci funtion added

* finbonacci funtion added

* finbonacci funtion added

* Update fibonacci.cpp
Co-authored-by: NChristian Clauss <cclauss@me.com>

math/fibonacci.cpp

+#include <iostream>
+#include <cassert>
+
+/* Calculate the the value on Fibonacci's sequence given an
+integer as input
+Fibonacci =  0, 1, 1, 2, 3, 5,
+             8, 13, 21, 34, 55,
+             89, 144, ...       */
+
+int fibonacci(uint n) {
+    /* If the input is 0 or 1 just return the same
+       This will set the first 2 values of the sequence */
+    if (n <= 1)
+        return n;
+
+    /* Add the last 2 values of the sequence to get next */
+    return fibonacci(n-1) + fibonacci(n-2);
+}
+
+int main() {
+    int n;
+    std::cin >> n;
+    assert(n >= 0);
+    std::cout << "F(" << n << ")= " << fibonacci(n) << std::endl;
+}