text: /**
   * Recursive Staircase Problem (Iterative Solution).
   *
   * @param {number} stairsNum - Number of stairs to climb on.
   * @return {number} - Number of ways to climb a staircase.
   */
  export default function recursiveStaircaseIT(stairsNum) {
    if (stairsNum <= 0) {
      // There is no way to go down - you climb the stairs only upwards.
      // Also you don't need to do anything to stay on the 0th step.
      return 0;
    }
  
    // Init the number of ways to get to the 0th, 1st and 2nd steps.
    const steps = [1, 2];
  
    if (stairsNum <= 2) {
      // Return the number of possible ways of how to get to the 1st or 2nd steps.
      return steps[stairsNum - 1];
    }
  
    // Calculate the number of ways to get to the n'th step based on previous ones.
    // Comparing to Dynamic Programming solution we don't store info for all the steps but
    // rather for two previous ones only.
    for (let currentStep = 3; currentStep <= stairsNum; currentStep += 1) {
      [steps[0], steps[1]] = [steps[1], steps[0] + steps[1]];
    }
  
    // Return possible ways to get to the requested step.
    return steps[1];
  }
  