//An efficient way to calculate nth fibonacci number faster and simpler than O(nlogn) method of matrix exponentiation //This works by using both recursion and dynamic programming. //as 93rd fibonacci exceeds 19 digits, which cannot be stored in a single long long variable, we can only use it till 92nd fibonacci //we can use it for 10000th fibonacci etc, if we implement bigintegers. //This algorithm works with the fact that nth fibonacci can easily found if we have already found n/2th or (n+1)/2th fibonacci //It is a property of fibonacci similar to matrix exponentiation. #include #include using namespace std; const long long MAX = 93; long long f[MAX] = {0}; long long fib(long long n) { if (n == 0) return 0; if (n == 1 || n == 2) return (f[n] = 1); if (f[n]) return f[n]; long long k = (n%2!=0)? (n+1)/2 : n/2; f[n] = (n%2!=0)? (fib(k)*fib(k) + fib(k-1)*fib(k-1)) : (2*fib(k-1) + fib(k))*fib(k); return f[n]; } int main() { //Main Function for(long long i=1;i<93;i++) { cout << i << " th fibonacci number is " << fib(i) << "\n"; } return 0; }