70. Climbing Stairs，70climbingstairs

70. Climbing Stairs，70climbingstairs

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?

遞推公式為f(n)=f(n-1)+f(n-2)

每個元素僅依賴於前兩個元素。

 1 class Solution {
 2 public:
 3     int climbStairs(int n) {
 4         int result[3] = {0,1,2};
 5         if(n < 3){
 6             return result[n];
 7         }
 8         
 9         int f1 = 1;
10         int f2 = 2;
11         int fn = 0;
12         
13         for(int i = 3; i <= n; i++ ){
14             fn = f1 + f2;
15             f1 = f2;
16             f2 = fn;
17         }
18         return fn;
19     }
20 };