牛客网真题2019-28-最少数量货物装箱问题

当我看到这一题的时候，就感觉可以利用数学方法解决，只要枚举前面十几个数就就能求解；因为贪心，一直减7到14，以内就肯定直接得到解。
最后用动态规划写出来，顺便复习一下动态规划，F(n)=F(n-7)+1 if:F(n-7)有解,F(n-5)+1 if:F(n-5)有解，F(n-3)+1 if:F(n-3)有解；

import java.util.Arrays;
import java.util.Scanner;

public class Main {


    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] temp = new int[]{-1, -1, -1, 1, -1, 1, 2, 1};
        if(n <= 7){
            System.out.println(temp[n]);
        }else{
            int[] ints = new int[n + 1];
            System.arraycopy(temp, 0, ints, 0, temp.length);
            for(int i = 8; i <= n; i++){
                int[] res = new int[3];
                res[0] = ints[i - 7] == -1 ? -1 : ints[i - 7] + 1;
                res[1] = ints[i - 5] == -1 ? -1 : ints[i - 5] + 1;
                res[2] = ints[i - 3] == -1 ? -1 : ints[i - 3] + 1;
                Arrays.sort(res);
                if(res[2] == -1){
                    ints[i] = -1;
                }else if(res[1] == -1){
                    ints[i] = res[2];
                }else if(res[0] == -1){
                    ints[i] = res[1];
                }else{
                    ints[i] = res[0];
                }
            }
            System.out.println(ints[n]);
        }

    }
}