why?

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <climits>
using namespace std;
int n,a[1001][1001],d[1001][1001];
int main() {
    scanf("%d",&n);
    for (int i = 0;i < n;i ++) {
        for (int j = 0;j <= i;j ++) {
            scanf("%d",&a[i][j]);
            if (i == 0) {
                d[0][0] = a[0][0];
            } else if (j == 0) {
                d[i][0] = a[i - 1][0] + a[i][j];
            } else {
                d[i][j] = max(d[i - 1][j],d[i - 1][j - 1]) + a[i][j];
            }
        }
    }
    sort (d[n - 1],d[n - 1] + n,greater<int>());
    printf("%d",d[n - 1][0]);
    return 0;
}