这怎么做呀，指导一下

#include <iostream>
#include <cstring>

using namespace std;

const int MAXN = 1000000;
int x = 2;
int a[MAXN+1];
int b[MAXN+1];
void init()
{
    memset(a, 0, sizeof(a));
    memset(b, 0, sizeof(b));
    a[0] = b[0] = 1;
    for(int i = 1; i < MAXN; ++i) {
        a[i] = a[i-1] + x++;
        b[i] = b[i-1] + a[i];
    }
}
int main(void)
{
    init();
    int n;
    scanf("%d", &n);
    printf("%d\n", b[n-1]);
    return 0;
}

#include <stdio.h>

int main()
{
int n;
scanf("%d",&n);
int sum = 0;
for(int i = 1;i<= n;i++)
{
for(int j = 1;j<=i;j++)
{
sum += j;
}
}
printf("%d\n",sum);

return 0;
}