[HIHO1050]树中的最长路（树直径）

  1     /*
  2     ━━━━━┒ギリギリ♂ eye！
  3     ┓┏┓┏┓┃キリキリ♂ mind！
  4     ┛┗┛┗┛┃＼○／
  5     ┓┏┓┏┓┃ /
  6     ┛┗┛┗┛┃ノ)
  7     ┓┏┓┏┓┃
  8     ┛┗┛┗┛┃
  9     ┓┏┓┏┓┃
 10     ┛┗┛┗┛┃
 11     ┓┏┓┏┓┃
 12     ┛┗┛┗┛┃
 13     ┓┏┓┏┓┃
 14     ┃┃┃┃┃┃
 15     ┻┻┻┻┻┻
 16     */
 17     #include <algorithm>
 18     #include <iostream>
 19     #include <iomanip>
 20     #include <cstring>
 21     #include <climits>
 22     #include <complex>
 23     #include <fstream>
 24     #include <cassert>
 25     #include <cstdio>
 26     #include <bitset>
 27     #include <vector>
 28     #include <deque>
 29     #include <queue>
 30     #include <stack>
 31     #include <ctime>
 32     #include <set>
 33     #include <map>
 34     #include <cmath>
 35     using namespace std;
 36     #define fr first
 37     #define sc second
 38     #define cl clear
 39     #define BUG puts("here!!!")
 40     #define W(a) while(a--)
 41     #define pb(a) push_back(a)
 42     #define Rint(a) scanf("%d", &a)
 43     #define Rll(a) scanf("%I64d", &a)
 44     #define Rs(a) scanf("%s", a)
 45     #define Cin(a) cin >> a
 46     #define FRead() freopen("in", "r", stdin)
 47     #define FWrite() freopen("out", "w", stdout)
 48     #define Rep(i, len) for(int i = 0; i < (len); i++)
 49     #define For(i, a, len) for(int i = (a); i < (len); i++)
 50     #define Cls(a) memset((a), 0, sizeof(a))
 51     #define Clr(a, x) memset((a), (x), sizeof(a))
 52     #define Full(a) memset((a), 0x7f7f7f, sizeof(a))
 53     #define lrt rt << 1
 54     #define rrt rt << 1 | 1
 55     #define pi 3.14159265359
 56     #define RT return
 57     #define lowbit(x) x & (-x)
 58     #define onecnt(x) __builtin_popcount(x)
 59     typedef long long LL;
 60     typedef long double LD;
 61     typedef unsigned long long ULL;
 62     typedef pair<int, int> pii;
 63     typedef pair<string, int> psi;
 64     typedef pair<LL, LL> pll;
 65     typedef map<string, int> msi;
 66     typedef vector<int> vi;
 67     typedef vector<LL> vl;
 68     typedef vector<vl> vvl;
 69     typedef vector<bool> vb;
 70 
 71     typedef struct Edge {
 72         int u, v, w;
 73         Edge() {}
 74         Edge(int uu, int vv, int ww) : u(uu), v(vv), w(ww) {}
 75     }Edge;
 76 
 77     const int maxn = 300030;
 78     int n;
 79     int ret;
 80     int length[maxn];
 81     bool vis[maxn];
 82     vector<Edge> edge[maxn];
 83 
 84     int bfs(int start) {
 85         Cls(vis); Cls(length);
 86         queue<int> q;
 87         q.push(start);
 88         vis[start] = 1;
 89         while(!q.empty()) {
 90             int u = q.front(); q.pop();
 91             Rep(i, edge[u].size()) {
 92                 int v = edge[u][i].v;
 93                 int w = edge[u][i].w;
 94                 if(vis[v]) continue;
 95                 vis[v] = 1;
 96                 length[v] = length[u] + w;
 97                 q.push(v);
 98             }
 99         }
100         int aim = 0;
101         For(i, 1, n) {
102             if(length[aim] < length[i]) aim = i;
103         }
104         return aim;
105     }
106 
107     int main() {
108         // FRead();
109         int u, v, w;
110         while(~Rint(n)) {
111             ret = 0x7f7f7f;
112             Rep(i, n+1) edge[i].clear();
113             Rep(i, n-1) {
114                 Rint(u); Rint(v);
115                 u--, v--, w = 1;
116                 edge[u].push_back(Edge(u, v, w));
117                 edge[v].push_back(Edge(v, u, w));
118             }
119             u = bfs(0);
120             v = bfs(u);
121             printf("%d\n", length[v]);
122         }    
123         RT 0;
124     }