climbStairs = (n) => {
    const isBaseCase = (n === 1);
    if (isBaseCase) return 1;

    let [ next, nextNext ] = [ 1, 2 ];

    for (let index = 3; (index <= n); index++) {/* Time O(N) */
        const temp = (next + nextNext);
        
        next = nextNext;
        nextNext = temp;
    }

    return nextNext;
}
DP - Fibonacci Number
Time O(N) | Space O(1)
https://leetcode.com/problems/climbing-stairs/
@param {number} n
@return {number}