70. 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?

DP方法：

step
= step[n-1]+step[n-2];

class Solution {
public:
int climbStairs(int n) {
vector<int> step(n,0);
step[0] = 1;
step[1] = 2;
for(int i = 2;i<n;i++){
step[i] = step[i-1]+step[i-2];
}
return step[n-1];
}
};