BZOJ 1509 [NOI2003]逃学的小孩

  1 #include <queue>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <iostream>
  5 
  6 using namespace std;
  7 
  8 const size_t Max_N(200050);
  9 const size_t Max_M(400050);
 10 
 11 size_t N;
 12 
 13 unsigned int Total;
 14 unsigned int Head[Max_N];
 15 unsigned int To[Max_M];
 16 unsigned long long int Weight[Max_M];
 17 unsigned int Next[Max_M];
 18 
 19 inline
 20 unsigned long long int Max(const unsigned long long int &a, const unsigned long long int &b)
 21 {
 22     return a > b ? a : b;
 23 }
 24 
 25 inline
 26 unsigned long long int Min(const unsigned long long int &a, const unsigned long long int &b)
 27 {
 28     return a < b ? a : b;
 29 }
 30 
 31 inline
 32 void Add_Edge(const size_t &s, const size_t &t, const unsigned long long int &w)
 33 {
 34     ++Total, To[Total] = t, Weight[Total] = w, Next[Total] = Head[s], Head[s] = Total;
 35 }
 36 
 37 void init()
 38 {
 39     unsigned int M;
 40     size_t u, v;
 41     unsigned long long int w;
 42     
 43     cin >> N >> M;
 44     while (M--)
 45     {
 46         cin >> u >> v >> w;
 47         Add_Edge(u, v, w);
 48         Add_Edge(v, u, w);
 49     }
 50 }
 51 
 52 struct node
 53 {
 54     node(const unsigned int &a = 0, const unsigned long long int &b = 0) : now(a), wsum(b) {}
 55     unsigned int now;
 56     unsigned long long int wsum;
 57 };
 58 
 59 size_t A, B;//树上最长链的两个端点
 60 
 61 unsigned long long int Dist_A[Max_N], Dist_B[Max_N];
 62 //分别存储A到各个点的距离和B到各个点的距离 
 63 
 64 unsigned long long int Now_Max;
 65 bool In_Q[Max_N];
 66 
 67 void BFS_1()
 68 {
 69     Now_Max = 0ULL;
 70     queue<node> Q;
 71     Q.push(node(1, 0ULL));
 72     In_Q[1] = true;
 73     node Top;
 74     while (Q.size())
 75     {
 76         Top = Q.front();
 77         Q.pop();
 78         if (Top.wsum > Now_Max)
 79         {
 80             Now_Max = Top.wsum;
 81             A = Top.now;
 82         }
 83         for (size_t i = Head[Top.now];i;i = Next[i])
 84             if (!In_Q[To[i]])
 85             {
 86                 In_Q[To[i]] = true;
 87                 Q.push(node(To[i], Top.wsum + Weight[i]));
 88             }    
 89     }
 90 }
 91 
 92 void BFS_2()
 93 {
 94     Now_Max = 0ULL;
 95     memset(In_Q, false, sizeof(In_Q));
 96     queue<node> Q;
 97     Q.push(node(A, 0ULL));
 98     In_Q[A] = true;
 99     node Top;
100     while (Q.size())
101     {
102         Top = Q.front();
103         Q.pop();
104         if (Top.wsum > Now_Max)
105         {
106             Now_Max = Top.wsum;
107             B = Top.now;
108         }
109         for (size_t i = Head[Top.now];i;i = Next[i])
110             if (!In_Q[To[i]])
111             {
112                 In_Q[To[i]] = true;
113                 Q.push(node(To[i], Top.wsum + Weight[i]));
114             }    
115     }
116 }
117 
118 void BFS_A()//求出A到各个点的距离
119 {
120     memset(In_Q, false, sizeof(In_Q));
121     queue<unsigned int> Q;
122     Q.push(A);
123     In_Q[A] = true;
124     unsigned int Top;
125     while (Q.size())
126     {
127         Top = Q.front();
128         Q.pop();
129         for (size_t i = Head[Top];i;i = Next[i])
130             if (!In_Q[To[i]])
131             {
132                 In_Q[To[i]] = true;
133                 Q.push(To[i]);
134                 Dist_A[To[i]] = Dist_A[Top] + Weight[i]; 
135             }    
136     }
137 }
138 
139 void BFS_B()//求出B到各个点的距离
140 {
141     memset(In_Q, false, sizeof(In_Q));
142     queue<unsigned int> Q;
143     Q.push(B);
144     In_Q[B] = true;
145     unsigned int Top;
146     while (Q.size())
147     {
148         Top = Q.front();
149         Q.pop();
150         for (size_t i = Head[Top];i;i = Next[i])
151             if (!In_Q[To[i]])
152             {
153                 In_Q[To[i]] = true;
154                 Q.push(To[i]);
155                 Dist_B[To[i]] = Dist_B[Top] + Weight[i]; 
156             }    
157     }
158 }
159 
160 void Longest_Path()//求出最长链相关信息 
161 {
162     BFS_1();
163     BFS_2();
164     BFS_A();
165     BFS_B();
166 }
167 
168 void Get_Ans()
169 {
170     unsigned long long int Ans(0ULL);
171     for (size_t i = 1;i <= N;++i)
172         Ans = Max(Ans, Now_Max + Min(Dist_A[i], Dist_B[i]));
173     cout << Ans << endl;
174 }
175 
176 int main()
177 {
178     init();
179     
180     Longest_Path();
181     
182     Get_Ans();
183     
184     return 0;
185 }