leetcode---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?

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