70. 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?

代码如下(方法一：超时)：

 public class Solution {
     public int climbStairs(int n) {
         if(n==1||n==2||n==3)
         return n;

         return climbStairs(n-1)+climbStairs(n-2);
     }
 }

方法二：AC

 public class Solution {
     public int climbStairs(int n) {
         if(n==1||n==2)
         return n;

         int num1=1,num2=2,sum=0;
         for(int i=3;i<=n;i++)
         { sum=num1+num2;
             num1=num2;
             num2=sum;
         }
         return sum;
     }
 }