下列C++代码片段中，能正确实现爬楼梯递推算法（每次可爬1或2级台阶，求到第n级的总爬法数）的是？

int climbStairs(int n) {
    if(n == 1) return 1;
    if(n == 2) return 2;
    return climbStairs(n-1) + climbStairs(n-2);
}

int climbStairs(int n) {
    if(n <= 2) return n;
    int a = 1, b = 2, c;
    for(int i = 3; i <= n; i++){
        c = a + b;
        a = b;
        b = c;
    }
    return b;
}

int climbStairs(int n) {
    if(n <= 2) return 3 - n;
    int a = 1, b = 2, c;
    for(int i = 3; i <= n; i++){
        c = a + b;
        a = b;
        b = c;
    }
    return b;
}

int climbStairs(int n) {
    if(n <= 2) return n;
    int a = 1, b = 2, c;
    for(int i = 3; i <= n; i++){
        c = a + b;
        a = c;
        b = c;
    }
    return b;
}