菲波那切数列

fib.js

const fib  = (num) => {
  if (num <= 0) return 0;
  if (num === 1 || num === 2) return 1;
  if (num > 2) {
    let prev = 1, next = 1;
    for (let i = 3; i <= num; i++) {
      let sum = prev + next;
      prev = next;
      next = sum;
    }
    return next;
  }
};