70. Climbing Stairs

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time: O(n)

space: O(1)

/**
 * @param {number} n
 * @return {number}
 */
var climbStairs = function(n, step = 2) {
    if (n < 2) return n;

    let first = 1;
    let second = 2;
    let cnt = 2;

    while (cnt < n) {
        const temp = second;
        second = first + second;
        first = temp;
        
        cnt++;
    }

    return second;
};