0070-climbing-stairs.c

int climbStairs(int n){
    if (n <= 3) {
        return n;
    }
    
    int n1 = 2;
    int n2 = 3;
    
    for (int i = 4; i <= n; i++) {
        int temp = n1 + n2;
        n1 = n2;
        n2 = temp;
    }
    return n2;
}