fib.js

/**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.

* @param {number} n
* @return {number}
*/
var fib = function (n) {
  if (n == 0) return 0;
  if (n == 1) return 1;

  let n1 = 0;
  let n2 = 1;

  for (let i = 2; i <= n; i++) {
    let n3 = n1 + n2;
    n1 = n2;
    n2 = n3;
  }
  return n2;
};