LeetCode70. 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?

爬楼梯问题是Fibonacci问题，只要记录前两步的台阶的方式，那么当前台阶的上来的方式就是前面两个的相加。

public class Solution {
public int climbStairs(int n) {
if(n==0){
return 0;
}
if(n ==1){
return 1;
}
if(n ==2){
return 2;
}
int first_before = 2;
int second_before =1 ;
int all_steps = 0;
for(int i= 3; i<= n; i++){
all_steps =first_before + second_before;
second_before = first_before;
first_before = all_steps;

}
return all_steps;
}
}

时间复杂度O(n)， 空间复杂度O(1)。