蓝桥杯 数学家的任务 知识点：枚举

9.数学家的任务 - 蓝桥云课 (lanqiao.cn)

暴力枚举

#include<bits/stdc++.h>
using namespace std;
#define int long long
int sum;
int a[] = {1,2,4,7};
signed main()
{
    for(int i=0;i<4;i++)
	{
		for (int j = 0; j < 4; j++)
		{
			for (int m = 0; m < 4; m++)
			{
				for (int n = 0; n < 4; n++)
				{
					sum += a[i] * 1000 + a[j] * 100 + a[m] * 10 + a[n];
				}
			}
		}
}

	cout << sum << endl;
  return 0;
}