public int countSimple(int n){
        int a = 1;
        int b = 1;
        
        for(int i=1; i < n; i++){
            int tmp = a;
        	a = a + b;
            b = tmp;
        }
        
        return a + b;
    }
http://www.geeksforgeeks.org/count-number-binary-strings-without-consecutive-1s/
It is really a straight up fibonacci series with values
1,2,3,5,8,13....
Look how we assign a[i] value of a[i-1] + b[i-1] and then b[i] becomes a[i]
src/com/interview/dynamic/CountNumberOfBinaryWithoutConsecutive1s.java