It just a Fibonacci sequence ： (0),(1),1,2,3,5,8,13,21,34 ......

We can find An = An-1 + An-2

```
int climbStairs(int n) {
int cnt = 2;
int first = 1 , second = 2;
int sum;
if( n == first) return first;
else if(n == second) return second;
else {
while(cnt < n){
sum = first + second;
first = second;
second = sum;
cnt++;
}
return sum;
}
}
```