18.上楼梯

import java.util.*;

public class GoUpstairs {
    public int countWays(int n) {
        int n1=1;
        int n2=2;
        int n3=4;
        int sum=0;
       if(n==1) return n1;
       if(n==2) return n2;
       if(n==3) return n3;
       for(int i=4;i<=n;i++){
            sum = ((n3+n2)%1000000007+n1)%1000000007;
            n1 = n2;
            n2 = n3;
            n3 = sum;
        }
        return sum;
    }

}