小杨正在爬楼梯，需要 n 阶才能到达楼顶，每次可以爬 1 阶或 2 阶，求小杨有多少种不同的方法可以爬到楼顶，横线上应填写()。
int climbStairs(int n){
if (n <= 2) return n;
int prev2 = 1;
int prev1 = 2;
int current = 0;
for (int i = 3; i <= n; ++i){
________________; // 在此处填入代码
}
return current;
}
prev2 = prev1; prev1 = current; current = prev1 + prev2;
current = prev1 + prev2; prev2 = prev1; prev1 = current;
current = prev1 + prev2; prev1 = current; prev2 = prev1;
prev1 = current; prev2 = prev1; current = prev1 + prev2;