[USACO13FEB] Tractor

题目链接

大家的 Blog 里面都是做过的题目，只有我的里面什么都没有

那我也开始写一点吧

刷着刷着 DP 不知怎的就出来一道这个题……用时 2 hours 后怒而删两个文件重构……

然后过了……过了……

至今不知道前两遍错在哪里……

Code - first time：连通块染色 +建图，然后二分，广搜判断

Code - second time：连通块染色 +建图，并查集维护连通块大小，做伪 Kruskal

Code - third time：二分，广搜判断

#include <queue>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;

const int maxn = 500 + 10;
const int maxk = 500 * 500 + 10;
int n, h[maxn][maxn], vis[maxn][maxn], flag;
int f[4][2] = { 0, 1, 1, 0, -1, 0, 0, -1 };

inline int Abs(int x) { return x > 0 ? x : -x; }

inline bool Breath_fs(int sx, int sy, int c) {
  queue<pair<int, int> > q;
  q.push(make_pair(sx, sy)), vis[sx][sy] = 1;
  int cnt = 1;
  while( !q.empty() ) {
    int x = q.front().first, y = q.front().second;
    for(int i = 0; i < 4; ++i) {
      int tx = x + f[i][0], ty = y + f[i][1];
      if( tx < 1 || tx > n || ty < 1 || ty > n ) continue;
      if( Abs(h[x][y] - h[tx][ty]) > c || vis[tx][ty] ) continue;
      ++cnt, vis[tx][ty] = 1, q.push(make_pair(tx, ty));
    }
    q.pop();
  }
  return cnt + 1 > ((n * n) >> 1) + ((n * n) & 1);
}

inline bool Check(int x) {
  memset(vis, 0, sizeof vis), flag = 0;
  for(int i = 1; i <= n; ++i) {
    for(int j = 1; (!flag && j <= n); ++j)
      if( !vis[i][j] ) flag = flag | Breath_fs(i, j, x);
    if( flag ) break;
  }
  return flag;
}

int main(int argc, char const *argv[])
{
  freopen("nanjolno.in", "r", stdin);
  freopen("nanjolno.out", "w", stdout);

  scanf("%d", &n);
  for(int i = 1; i <= n; ++i) for(int j = 1; j <= n; ++j) scanf("%d", &h[i][j]);
  int lef = 0, righ = 1000000, mid = (lef + righ) >> 1, ans = 0;
  for( ; lef <= righ; mid = (lef + righ) >> 1)
    Check(mid) ? ans = mid, righ = mid - 1 : lef = mid + 1;
  printf("%d
", ans);

  fclose(stdin), fclose(stdout);
  return 0;
}

—— 晨意微寒秋渐深，侧畔无事俏佳人。