18. Up Stairs

Title Description

There is a child on the stairs, the stairs have n steps, the child can be on the 1, 2, 3 order. Please implement a method to calculate how many ways a child has to go upstairs. In order to prevent overflow, please set the result mod 1000000007

Given a positive integer int n, return a number representing the number of ways to go upstairs. Guaranteed n is less than or equal to 100000.

Test Sample: 1

Returns: 1
The code is as follows:

Import java.util.*;p Ublic 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;    }}

　　

18. Up Stairs