假设你正在爬楼梯，需要n步你才能到达顶部。但每次你只能爬一步或者两步，你能有多少种不同的方法爬到楼顶部？

样例
Example 1:
Input: n = 3
Output: 3

Explanation:
1) 1, 1, 1
2) 1, 2
3) 2, 1
total 3.

Example 2:
Input: n = 1
Output: 1

Explanation:  
only 1 way.

class Solution {
public:
    /**
     * @param n: An integer
     * @return: An integer
     */
    int climbStairs(int n) {
        // write your code here
       if(n<3) return n;
       else 
       {
            int num1=1;
            int num2=2;
            int cur=0;
            for(int i=3;i<=n;i++)
            {
               cur=num1+num2;
               num1=num2;
               num2=cur;
            }
            return cur;
       }
    }
};

class Solution {
public:
    /**
     * @param n: An integer
     * @return: An integer
     */
    int climbStairs(int n) {
        // write your code here
       vector<int> result;
    	result.push_back(0);
    	result.push_back(1);
    	result.push_back(2);
    	if (n > 2)
    	{
    		for (int i = 3; i <= n; i++)
    		{
       			result.push_back(result[i - 1] + result[i - 2]);
    		}
    	}
    	return result[n];
    }
};```