UVALive - 3211 - Now or later（图论——2-SAT）

  1 #include <bits/stdc++.h>
  2 
  3 using namespace std;
  4 
  5 #define REP(i, n) for (int i = 1; i <= (n); i++)
  6 #define sqr(x) ((x) * (x))
  7 
  8 const int maxn = 2000 + 10;
  9 const int maxm = 30 + 10;
 10 const int maxs = 10000 + 10;
 11 
 12 typedef long long LL;
 13 typedef pair<int, int> pii;
 14 typedef pair<double, double> pdd;
 15 
 16 const LL unit = 1LL;
 17 const int INF = 0x3f3f3f3f;
 18 const LL mod = 1000000007;
 19 const double eps = 1e-14;
 20 const double inf = 1e15;
 21 const double pi = acos(-1.0);
 22 
 23 struct TwoSAT
 24 {
 25     int n;
 26     vector<int> G[maxn * 2];
 27     bool mark[maxn * 2];
 28     int S[maxn * 2], c;
 29 
 30     bool dfs(int x)
 31     {
 32         if (mark[x ^ 1])
 33             return false;
 34         if (mark[x])
 35             return true;
 36         mark[x] = true;
 37         S[c++] = x;
 38         for (auto v : G[x])
 39         {
 40             if (!dfs(v))
 41                 return false;
 42         }
 43         return true;
 44     }
 45 
 46     void init(int n)
 47     {
 48         this->n = n;
 49         for (int i = 0; i < n * 2; i++)
 50         {
 51             G[i].clear();
 52         }
 53         memset(mark, 0, sizeof(mark));
 54     }
 55 
 56     void add_clause(int x, int xval, int y, int yval)
 57     {
 58         x = x * 2 + xval;
 59         y = y * 2 + yval;
 60         G[x ^ 1].push_back(y);
 61         G[y ^ 1].push_back(x);
 62     }
 63 
 64     bool solve()
 65     {
 66         for (int i = 0; i < n * 2; i += 2)
 67         {
 68             if (!mark[i] && !mark[i + 1])
 69             {
 70                 c = 0;
 71                 if (!dfs(i))
 72                 {
 73                     while (c > 0)
 74                     {
 75                         mark[S[--c]] = false;
 76                     }
 77                     if (!dfs(i + 1))
 78                         return false;
 79                 }
 80             }
 81         }
 82         return true;
 83     }
 84 };
 85 
 86 TwoSAT solver;
 87 
 88 int n, T[maxn][2];
 89 
 90 bool Judge(int lim)
 91 {
 92     solver.init(n);
 93     for (int i = 0; i < n; i++)
 94     {
 95         for (int a = 0; a < 2; a++)
 96         {
 97             for (int j = i + 1; j < n; j++)
 98             {
 99                 for (int b = 0; b < 2; b++)
100                 {
101                     if (abs(T[i][a] - T[j][b]) < lim)
102                     {
103                         solver.add_clause(i, a ^ 1, j, b ^ 1);
104                     }
105                 }
106             }
107         }
108     }
109     return solver.solve();
110 }
111 
112 main()
113 {
114     ios::sync_with_stdio(false);
115     cin.tie(0);
116     //freopen("input.txt", "r", stdin);
117     //freopen("output.txt", "w", stdout);
118     while (cin >> n && n)
119     {
120         int le = 0, ri = 0;
121         for (int i = 0; i < n; i++)
122         {
123             for (int j = 0; j < 2; j++)
124             {
125                 cin >> T[i][j];
126                 ri = max(ri, T[i][j]);
127             }
128         }
129 
130         int ans = 0;
131         while (le <= ri)
132         {
133             int mid = (le + ri) >> 1;
134             if (Judge(mid))
135             {
136                 ans = mid;
137                 le = mid + 1;
138             }
139             else
140             {
141                 ri = mid - 1;
142             }
143         }
144         cout << ans << endl;
145     }
146     return 0;
147 }