boj13558.c

#include <stdio.h>
#include <stdlib.h>
#define VM 30000

void solve13558(void)
{ typedef long long unsigned int llu_t;
  typedef short int val_t;
  int count_vi[VM], count_vk[VM];
  val_t ary[100000];
  llu_t res = 0;
  size_t len;
  memset(count_vi, 0x0, sizeof(count_vi));
  memset(count_vk, 0x0, sizeof(count_vk));
  scanf("%zd", &len);
  for (size_t n = 0; n < len; n++)
  { int input;
    scanf("%d", &input);
    ary[n] = input - 1;
  }
  for (size_t k = 0; k < len; k++)
  { const val_t vk = ary[k];
    count_vk[vk]++;
  }
  for (size_t j = 0; j < len; j++)
  { const val_t vj = ary[j];
    count_vk[vj]--;
    if (2 * vj < VM)
    { for (val_t vi = 0; vi <= 2 * vj; vi++)
      { const val_t vk = 2 * vj - vi;
        res += (llu_t)count_vi[vi] * (llu_t)count_vk[vk];
      }
    }
    else
    { for (val_t vk = 2 * vj - VM + 1; vk <= VM - 1; vk++)
      { const val_t vi = 2 * vj - vk;
        res += (llu_t)count_vi[vi] * (llu_t)count_vk[vk];
      }
    }
    count_vi[vj]++;
  }
  printf("%llu\n", res);
}

int main(void)
{ solve13558();
  return 0;
}