Constructing Roads hdu1102

http://acm.hdu.edu.cn/showproblem.php?pid=1102

// 是最小產生數的變題，一開始題目沒有看仔細which means the road between village a and village b has been built.
//這句話沒有注意，看題目要仔細啊
#include<iostream>//2242938 2010-03-24 20:03:34 Accepted 1102 31MS 344K 1200 B C++ 悔惜晟 
#include<algorithm>
#include<cstdio>
#include<cmath>
using namespace std;
int s[9591];
 
 struct stu
 {
  int x;
  int y;
  int dis;
 }df[9591];
 
 int cmp(stu a, stu b)
 {
  return a.dis < b.dis;
 }
 int find(int a)
 {
  int r = a;
  while(r != s[r])
   r = s[r];
  return r;
 }
 
 void merge(int a, int b)
 {
  if(a > b)
   s[b] = a;
  else
   s[a] = b;
 }
 
int main()
{
 int N, q, aa[102][102], i , j, n, m, sum;
 while(scanf("%d", &N) != EOF)
 {
  for(i = 1; i <= N; i++)
   for(j = 1; j<= N; j++)
    scanf("%d", &aa[i][j]);
  scanf("%d", &q);
  for(i = 0; i < q; i++)
  {
   scanf("%d%d", &n, &m);
   df[i].x = n;
   df[i].y = m;
   df[i].dis = 0;//這裡表示已經修好了的
   aa[n][m] = 0;//一開始這裡寫成aa[i][j] = 0;造成數組越界了
  }
  for(i = 1 ; i <= N-1; i++)
   for(j = i+1; j <=N ; j++)
   {
    if(aa[i][j] != 0)
    {
     df[q].x = i;
     df[q].y = j;
     df[q].dis = aa[i][j];
     q++;
    }
   }
  sort(df, df+q, cmp);
  sum = 0;
  for(i = 1; i <= N; i++)
   s[i] = i;
  for(i = 0; i < q; i++)
  {
   int a = find(df[i].x);
   int b = find(df[i].y);
   if(a != b)
   {
    sum += df[i].dis;
    merge(a, b);
   }
  }
  printf("%d/n", sum);
 }
}