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;
}

{{ select(1) }}

• 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;