[second]Climbing Stairs

f(n) = f(n-1)+f(n-2) DP

    int climbStairs(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n<=2)
            return n;
        int a = 1,b = 2;
        int cnt;
        for(int i=3;i<=n;i++)
        {
            cnt = a+b;
            a = b;
            b = cnt;
        }
        
        return cnt;
        
    }