LeetCode 70 Climbing Stairs

int climbStairs(int n) {
    if(n<=0)
        return 0;
    if(n==1)
        return 1;
    if(n==2)
        return 2;
    return climbStairs(n-1)+climbStairs(n-2); //最后必然是通过1步或者2步到达第n层 
}

int climbStairs(int n) {
    if (n == 0 || n == 1)
      return 1;
    int pre = 1;
    int current = 1;
    for (int i = 2; i <= n; i++) {
      int temp = current + pre;
      pre = current;
      current = temp;
    }
    return current;
}