前一半AC后一半RE是怎么回事

#define _CRT_SECURE_NO_WARNINGS 1;
#include<cstdio>
using namespace std;
int n, arr[2][500000], flag = 0;
long long sum = 0;
void sort(int, int, int, int);
void mergesort();
int main() {
	scanf("%d", &n);
	int i;
	for (i = 0; i < n; i++) scanf("%d", &arr[0][i]);
	for (i = n; i < 500000; i++) arr[0][i] = arr[1][i] = 0x3f3f3f3f;
	mergesort();
	printf("%lld", sum);
	return 0;
}
void sort(int arr1, int arr2, int n1, int n2) {
	int i = 0, j = 0, k = 0;
	while (i < n1 && j < n2) {
		if (arr[flag][arr1 + i] > arr[flag][arr2 + j]) {
			arr[(flag + 1) % 2][arr1 + k] = arr[flag][arr1 + i];
			k++;
			sum += (long long)(n2 - j);
			i++;
		}
		if (arr[flag][arr1 + i] <= arr[flag][arr2 + j]) {
			arr[(flag + 1) % 2][arr1 + k] = arr[flag][arr2 + j];
			k++;
			j++;
		}
	}
	if (i == n1) {
		while (j < n2) {
			arr[(flag + 1) % 2][arr1 + k] = arr[flag][arr2 + j];
			j++;
			k++;
		}
	}
	else {
		while (i < n1) {
			arr[(flag + 1) % 2][arr1 + k] = arr[flag][arr1 + i];
			i++;
			k++;
		}
	}
}
void mergesort() {
	int l, i;
	for (l = 1;l < n - 1; l *= 2) {
		for (i = 0; i <= n - 2 * l; i += 2 * l) {
			sort(i, i + l, l, l);
		}
		sort(i, i + l, l, n - i - l);
        flag = (flag + 1) % 2;
	}
}