Climbing Stairs

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

即求菲波拉契数列，第一次

public int climbStairs(int
n) {
int
ans = 1;
if (n == 1)
return ans;
if (n == 2)
return ans;
ans = climbStairs(n-1) + climbStairs(n-2);
return ans;
}
超时，需要用DP来解决。

public int climbStairs(int
n) {
int
ans = 1;
int current = 1,prev = 2;
if (n == 1)
return current;
if (n == 2)
return prev;
for (int
i = 2; i < n;
i++){
ans =
current + prev;
current =
prev;
prev =
ans;
}
return ans;
}