text: const store = {};
  /**
   * @param {number} n
   * @return {number}
   */
  const climbStairs = function(n) {
    if (store[n]) return store[n];
    let p = 0, q = 0, r = 1;
    for (let i = 1; i <= n; ++i) {
      [p, q, r] = [q, r, r + q];
    }
    return store[n] = r;
  };
  