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?

Note: Given n will be a positive integer.

public class Solution {
public int climbStairs(int n) {
int a=1;
int b=2;
if(n==1)return a;
if(n==2)return b;
for(int i=3;i<=n;i++){
int index=a;
a=b;
b=index+a;
}
return b;
}
}