hihoCoder#1141 二分·归并排序之逆序对

原题地址

又是一道WA成狗的题，最后发现原来是结果溢出了。。

代码：

#include <iostream>
#include <cstring>

using namespace std;

#define MAX_N 100008

int N;
long long a[MAX_N];
long long b[MAX_N];

long long count(int l, int r) {
  if (l >= r)
    return 0;

  long long res = 0;
  int m = (l + r) / 2;
  res += count(l, m);
  res += count(m + 1, r);
  int i = l;
  int j = m + 1;
  int k = 0;
  while (i <= m && j <= r) {
    if (a[i] <= a[j]) {
      b[k++] = a[i++];
    }
    else {
      res += (m - i + 1);
      b[k++] = a[j++];
    }
  }
  while (i <= m)
    b[k++] = a[i++];
  while (j <= r)
    b[k++] = a[j++];

  while (k--)
    a[l + k] = b[k];
  return res;
}

int main() {
  scanf("%d", &N);
  for (int i = 0; i < N; i++)
    scanf("%lld", &(a[i]));
  printf("%lld
", count(0, N - 1));
  return 0;
}