Y病毒

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int delay = in.nextInt();
        int forget = in.nextInt();
        int arr[] = new int[n+1];
        int brr[] = new int[n+1];

        //对特殊情况的判断
        if (forget<=delay){//无法繁殖情况
            if (n <= forget+1)//forget为几天后
                System.out.println(1);
            else
                System.out.println(0);
            return;
        }
        for (int i = 1; i <= delay; i++) {
            if (i==1)
                arr[1] = 1;//第一天生成一个病毒
            else
                arr[i] = 0;
        }
        //使用数组记录每天增长的数量
        //1+delay天后开始繁殖
        for (int i = delay+1 ; i  <= n; i++) {
            int preSum = 0;
            //for (int j = i-1-(forget-delay); j < i-1; j++) {//求解目前在繁殖期的所有病毒
            for (int j = i-forget+1; j < i-delay+1; j++) {//求解目前在繁殖期的所有病毒
                //从delay --- forget 是他的繁殖时间长度，即求解 i-1-2 === i-2
                if(j<1)
                    continue;//前面的左边界可能会越界
                preSum += arr[j];
            }
            arr [i] =preSum;//此时要算有繁殖能力的病毒总数
        }
        //用brr数组存储该时刻消失的病毒
        for (int i = 1; i <= forget; i++) {
            brr[i] = 0;
        }
        for (int i = forget+1 ; i  <= n; i++) {//forget后开始消失
            brr [i] =arr[i-forget];//此时要算正好forget天前生成的病毒
        }
        int ans = 0;
        for (int i = 1; i <= n; i++) {
            ans+=(arr[i]-brr[i])%1000000007;
        }
        System.out.println(ans);
    }
}

测试用例：

6 2 4
5