bzoj2657: [Zjoi2012]旅游(journey)

求树的直径 真是太神辣

 #include<cstdio>
 #include<cstring>
 #include<cstdlib>
 #include<algorithm>
 #include<iostream>

 using namespace std;

 template<typename Q> Q &read(Q &x) {
     static char c, f;
     for(f = ; c = getchar(), !isdigit(c); ) if(c == '-') f = ;
     for(x = ; isdigit(c); c = getchar()) x = x *  + c - '';
     if(f) x = -x; return x;
 }
 template<typename Q> Q read() {
     static Q x; read(x); return x;
 }

 const int N();
 int n;

 bool neighbor(int a, int b) {
     return abs(a - b) ==  || abs(a - b) == n - ;
 }

 #include<map>
 map<pair<int, int>, int> mp;
 int ID(int u, int v) {
     if(u > v) swap(u, v);
     if(mp.count(make_pair(u, v))) return mp[make_pair(u, v)];
     return mp[make_pair(u, v)] = mp.size();
 }

 int no[N];

 #include<vector>
 vector<int> G[N];

 void AddEdge(int u, int v) {
     G[u].push_back(v);
     G[v].push_back(u);
 }

 int q[N], ql, qr, dep[N];

 void BFS(int s) {
     ql = qr = ;
     memset(dep, , sizeof dep);
     q[qr++] = s, dep[s] = ;
     while(ql < qr) {
         int u = q[ql++];
         for(unsigned i = ; i < G[u].size(); i++) {
             int v = G[u][i];
             if(!dep[v]) {
                 q[qr++] = v;
                 dep[v] = dep[u] + ;
             }
         }
     }
 }

 int main() {
 #ifdef DEBUG
     freopen("in.txt", "r", stdin);
     freopen("out.txt", "w", stdout);
 #endif

     scanf("%d", &n);
     for(int i = ; i <= n - ; i++) {
         int a[];
         for(int j = ; j < ; j++) scanf("%d", a + j);
         a[] = a[];
         for(int j = ; j < ; j++) {
             if(!neighbor(a[j], a[j+])) {
                 int id = ID(a[j], a[j+]);
                 if(no[id]) AddEdge(no[id], i);
                 else no[id] = i;
             }
         }
     }

     BFS(), BFS(q[qr - ]);
     printf("%d\n", dep[q[qr - ]]);

     return ;
 }