bzoj2657: [Zjoi2012]旅游(journey)

求树的直径 真是太神辣

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<cstdlib>
 4 #include<algorithm>
 5 #include<iostream>
 6 
 7 using namespace std;
 8 
 9 template<typename Q> Q &read(Q &x) {
10     static char c, f;
11     for(f = 0; c = getchar(), !isdigit(c); ) if(c == '-') f = 1;
12     for(x = 0; isdigit(c); c = getchar()) x = x * 10 + c - '0';
13     if(f) x = -x; return x;
14 }
15 template<typename Q> Q read() {
16     static Q x; read(x); return x;
17 }
18 
19 const int N(200000);
20 int n;
21 
22 bool neighbor(int a, int b) {
23     return abs(a - b) == 1 || abs(a - b) == n - 1;
24 }
25 
26 #include<map>
27 map<pair<int, int>, int> mp;
28 int ID(int u, int v) {
29     if(u > v) swap(u, v);
30     if(mp.count(make_pair(u, v))) return mp[make_pair(u, v)];
31     return mp[make_pair(u, v)] = mp.size();
32 }
33 
34 int no[N];
35 
36 #include<vector>
37 vector<int> G[N];
38 
39 void AddEdge(int u, int v) {
40     G[u].push_back(v);
41     G[v].push_back(u);
42 }
43 
44 int q[N], ql, qr, dep[N];
45 
46 void BFS(int s) {
47     ql = qr = 0;
48     memset(dep, 0, sizeof dep);
49     q[qr++] = s, dep[s] = 1;
50     while(ql < qr) {
51         int u = q[ql++];
52         for(unsigned i = 0; i < G[u].size(); i++) {
53             int v = G[u][i];
54             if(!dep[v]) {
55                 q[qr++] = v;
56                 dep[v] = dep[u] + 1;
57             }
58         }
59     }
60 }
61 
62 int main() {
63 #ifdef DEBUG
64     freopen("in.txt", "r", stdin);
65     freopen("out.txt", "w", stdout);
66 #endif
67     
68     scanf("%d", &n);
69     for(int i = 1; i <= n - 2; i++) {
70         int a[4];
71         for(int j = 0; j < 3; j++) scanf("%d", a + j);
72         a[3] = a[0];
73         for(int j = 0; j < 3; j++) {
74             if(!neighbor(a[j], a[j+1])) {
75                 int id = ID(a[j], a[j+1]);
76                 if(no[id]) AddEdge(no[id], i);
77                 else no[id] = i;
78             }
79         }
80     }
81     
82     BFS(1), BFS(q[qr - 1]);
83     printf("%d\n", dep[q[qr - 1]]);
84     
85     return 0;
86 }
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posted @ 2016-02-26 19:08  Showson  阅读(138)  评论(0)  编辑  收藏  举报