Climbing Stairs

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?

 

 1 public class Solution {

 2     public int climbStairs(int n) {

 3         // IMPORTANT: Please reset any member data you declared, as

 4         // the same Solution instance will be reused for each test case.

 5         // s(n) = s(n - 1) + s(n - 2)

 6         if(n < 1)return 0;

 7         else if(n == 1) return 1;

 8         else if(n == 2) return 2;

 9         int first = 1;

10         int second = 2;

11         int result = 0;

12         for(int i = 3; i <= n; i ++){

13             result = first + second;

14             first = second;

15             second = result;

16         }

17         return result;

18     }

19 }

 第三遍：

use only 2 constant space：

 1 public class Solution {

 2     public int climbStairs(int n) {

 3         if(n < 1) return 0;

 4         int[] result = new int[] { 1, 2 };

 5         for(int i = 2; i < n; i ++){

 6             int tmp = result[0] + result[1];

 7             result[i % 2] = tmp;

 8         }

 9         return result[(n - 1) % 2];

10     }

11 }