LC509.cpp

/*509. Fibonacci Number

The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

F(0) = 0, F(1) = 1
F(n) = F(n - 1) + F(n - 2), for n > 1.
Given n, calculate F(n).

 

Example 1:

Input: n = 2
Output: 1
Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.
Example 2:

Input: n = 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.
Example 3:

Input: n = 4
Output: 3
Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.
 

Constraints:

0 <= n <= 30 */

class Solution {
public:
    int fib(int n) {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        
        if (n < 2) 
            return n;
        int a = 0, b = 1, current;
        
        for (int i = 1; i < n; i++)
            current = a, a = b, b += current;
        return b;
    }
};