climbStairs = (n, index = 0) => {
    const isBaseCase1 = (n < index);
    if (isBaseCase1) return 0;

    const isBaseCase2 = (index === n);
    if (isBaseCase2) return 1;

    const [ next, nextNext ] = [ (index + 1), (index + 2) ];
    const left = climbStairs(n, next);     /* Time O(2^N) | Space O(N) */
    const right = climbStairs(n, nextNext);/* Time O(2^N) | Space O(N) */

    return (left + right);
}
Brute Force - DFS
Time O(2^N) | Space O(N)
https://leetcode.com/problems/climbing-stairs/
@param {number} n
@return {number}