70. Climbing Stairs

https://leetcode.com/problems/climbing-stairs/description/
Solution 1.
    int climbStairs(int n) {
        if(n<=3) return n;
        vector<int> ways(n,0);
        ways[0] = 1;
        ways[1] = 2;
        ways[2] = 3;
        for(int i=3;i<n;i++) {
            ways[i] = ways[i-1] + ways[i-2];
        }
        return ways[n-1];
    }