70. Climbing Stairs

题目：

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?
解题思路：
动态规划

class Solution {
public:
int climbStairs(int n) {
if(n<=2)return n;
vector<int> res(n+1,0);
res[1]=1;  res[2]=2;
for(int i=3;i<=n;i++)
res[i] = res[i-1]+res[i-2];
return res
;
}
};