70.climbing-stairs.java

/*
 * @lc app=leetcode id=70 lang=java
 *
 * [70] Climbing Stairs
 *
 * https://leetcode.com/problems/climbing-stairs/description/
 *
 * algorithms
 * Easy (43.48%)
 * Total Accepted:    381.1K
 * Total Submissions: 869K
 * Testcase Example:  '2'
 *
 * You are climbing a stair case. It takes n steps to reach to the top.
 *
 * Each time you can either climb 1 or 2 steps. In how many distinct ways can
 * you climb to the top?
 *
 * Note: Given n will be a positive integer.
 *
 * Example 1:
 *
 *
 * Input: 2
 * Output: 2
 * Explanation: There are two ways to climb to the top.
 * 1. 1 step + 1 step
 * 2. 2 steps
 *
 *
 * Example 2:
 *
 *
 * Input: 3
 * Output: 3
 * Explanation: There are three ways to climb to the top.
 * 1. 1 step + 1 step + 1 step
 * 2. 1 step + 2 steps
 * 3. 2 steps + 1 step
 *
 *
 */
class Solution {
  public int climbStairs(int n) {
    if (n <= 0) return 0;
    if (n == 1) return 1;
    if (n == 2) return 2;

    int one_step_before = 2;
    int two_steps_before = 1;
    int result = 0;

    for (int i = 2; i < n; i++) {
      result = one_step_before + two_steps_before;
      two_steps_before = one_step_before;
      one_step_before = result;
    }

    return result;
  }
}