0509. Fibonacci Number.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: 2
// Output: 1
// Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1.

// Example 2:

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

// Example 3:

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

// Note:

// 0 ≤ N ≤ 30.


// 参见 0070
/**
 * @param {number} N
 * @return {number}
 */
var fib = function (N) {
  let res
  let first = 0
  let second = 1
  if (N >= 0 && N < 2) {
    return N
  } else if (N <= 30) {
    for (let i = 1; i < N; i++) {
      res = first + second
      first = second
      second = res
    }
  }
  return res
}
// Runtime: 52 ms, faster than 78.21 % of JavaScript online submissions for Fibonacci Number.
// Memory Usage: 33.8 MB, less than 81.82 % of JavaScript online submissions for Fibonacci Number.