给n个点， 初始有m条边， q个操作。
每个操作有两种， 1是询问点x所在的连通块内的最长路径， 就是树的直径。 2是将x, y所在的两个连通块连接起来，并且要合并之后的树的直径最小，如果属于同一个连通块就忽视这个操作。
先dfs出每个连通块的初始直径， 然后合并的话就是len[x] = max( (len[x]+1)/2+(len[y]+1)/2+1, max(len[x], len[y])); 然后搞一搞就好了。
一开始写bfs写挫了一直超时， 只好改成dfs......
1 #include<bits/stdc++.h> 2 using namespace std; 3 #define pb(x) push_back(x) 4 #define ll long long 5 #define mk(x, y) make_pair(x, y) 6 #define lson l, m, rt<<1 7 #define mem(a) memset(a, 0, sizeof(a)) 8 #define rson m+1, r, rt<<1|1 9 #define mem1(a) memset(a, -1, sizeof(a)) 10 #define mem2(a) memset(a, 0x3f, sizeof(a)) 11 #define rep(i, a, n) for(int i = a; i<n; i++) 12 #define ull unsigned long long 13 typedef pair<int, int> pll; 14 const double PI = acos(-1.0); 15 const double eps = 1e-8; 16 const int mod = 1e9+7; 17 const int inf = 1061109567; 18 const int dir[][2] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} }; 19 const int maxn = 3e5+5; 20 int f[maxn], num[maxn], head[maxn*2], dis[maxn], cnt, q[maxn*2]; 21 struct node 22 { 23 int to, nextt; 24 }e[maxn*2]; 25 void add(int u, int v) { 26 e[cnt].to = v; 27 e[cnt].nextt = head[u]; 28 head[u] = cnt++; 29 } 30 int findd(int u) { 31 return f[u] == u?u:f[u] = findd(f[u]); 32 } 33 void unionn(int x, int y) { 34 x = findd(x); 35 y = findd(y); 36 if(x == y) 37 return ; 38 f[y] = x; 39 int now = (1+num[x])/2+(num[y]+1)/2+1; 40 num[x] = max(now, max(num[x], num[y])); 41 } 42 int maxx = 0, pos = -1; 43 int dfs(int u, int fa, int d) { 44 if(d>maxx) { 45 maxx = d; 46 pos = u; 47 } 48 for(int i = head[u]; ~i; i = e[i].nextt) { 49 int v = e[i].to; 50 if(v == fa) 51 continue; 52 dfs(v, u, d+1); 53 } 54 } 55 template<typename __ll> 56 inline void read(__ll &m) 57 { 58 __ll x=0,f=1;char ch=getchar(); 59 while(!(ch>='0'&&ch<='9')){if(ch=='-')f=-1;ch=getchar();} 60 while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();} 61 m=x*f; 62 } 63 int main() 64 { 65 int n, m, q, x, y; 66 cin>>n>>m>>q; 67 mem1(head); 68 for(int i = 1; i<=n; i++) { 69 f[i] = i; 70 num[i] = 0; 71 } 72 while(m--) { 73 read(x); read(y); 74 add(x, y); 75 add(y, x); 76 x = findd(x); y = findd(y); 77 f[x] = y; 78 } 79 for(int i = 1; i<=n; i++) { 80 if(f[i] == i) { 81 dfs(i, -1, 0); 82 maxx = 0; 83 if(pos == -1) 84 continue; 85 dfs(pos, -1, 0); 86 num[i] = maxx; 87 maxx = 0; 88 pos = -1; 89 } 90 } 91 while(q--) { 92 int sign; 93 read(sign); 94 if(sign == 1) { 95 read(x); 96 printf("%d ", num[findd(x)]); 97 } else { 98 read(x); read(y); 99 unionn(x, y); 100 } 101 } 102 }