100197. Three Steps Problem LCCI

难度: Easy

题目描述

A child is running up a staircase with n steps and can hop either 1 step, 2 steps, or 3 steps at a time. Implement a method to count how many possible ways the child can run up the stairs. The result may be large, so return it modulo 1000000007.

Example1:

 Input: n = 3 
 Output: 4

Example2:

 Input: n = 5
 Output: 13

Note:

1. 1 <= n <= 1000000

解题思路

代码实现

解决方案

java

class Solution {
    public int waysToStep(int n) {

        if (n == 1) {
            return 1;

        }
        if (n == 2) {
            return 2;
        }
        if (n == 3) {
            return 4;
        }
        
        int [] dp = new int[n+1];
        dp[0]=1;
        dp[1]=1;
        dp[2]=2;
        for(int i=3;i<=n;i++){
            dp[i] = ((dp[i-1]+dp[i-2])%1000000007+dp[i-3])%1000000007;
             
        }

        return dp[n]%1000000007;

    }
}