P1967-开车旅行

  1 #include <bits/stdc++.h>
  2 #define _for(i,a,b) for(int i = (a);i < b;i ++)
  3 #define _rep(i,a,b) for(int i = (a);i > b;i --)
  4 #define INF 0x3f3f3f3f
  5 #define pb push_back
  6 #define maxn 500003
  7 #define SWAP(x,y) x^=y,y^=x,x^=y
  8 typedef long long ll;
  9 using namespace std;
 10 
 11 inline ll read()
 12 {
 13     ll ans = 0;
 14     char ch = getchar(), last = ' ';
 15     while(!isdigit(ch)) last = ch, ch = getchar();
 16     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
 17     if(last == '-') ans = -ans;
 18     return ans;
 19 }
 20 inline void write(ll x)
 21 {
 22     if(x < 0) x = -x, putchar('-');
 23     if(x >= 10) write(x / 10);
 24     putchar(x % 10 + '0'); 
 25 }
 26 
 27 int N,M,S;
 28 vector<int> G[maxn*2];
 29 bool vis[maxn];
 30 int depth[maxn];
 31 int fa[maxn][25];
 32 int w[maxn][25];
 33 
 34 
 35 int par[maxn]; //父亲 
 36 int high[maxn]; //树的高度
 37 
 38 void init(int n)
 39 {
 40     _for(i,1,n+1)
 41     {
 42         par[i] = i;
 43         high[i] = 0;
 44     }
 45 } 
 46 
 47 int find(int x)
 48 {
 49     return par[x] == x ? x : par[x] = find(par[x]);
 50 }
 51 
 52 void unite(int x,int y)
 53 {
 54     x = find(x);y = find(y);
 55     if(x==y) return ;
 56     
 57     if(high[x]<high[y])
 58         par[x] = y;
 59     else
 60     {
 61         par[y] = x;
 62         if(high[x]==high[y])
 63             high[x] ++;
 64     }
 65 }
 66 
 67 bool same(int x,int y)
 68 {
 69     return find(x) == find(y); 
 70 }
 71 
 72 struct tedge
 73 {
 74     int u;
 75     int v;
 76     int cost;
 77 };
 78 int V,E;
 79 vector<tedge> es;
 80 vector<tedge> GG[maxn]; 
 81 bool cmp(const tedge& a,const tedge& b)
 82 {
 83     return a.cost > b.cost;
 84 } 
 85 void MST()
 86 {    
 87     sort(es.begin(),es.end(),cmp);
 88     init(V);
 89     _for(i,0,E)
 90     {
 91         tedge e = es[i];
 92         if(!same(e.u,e.v))
 93         {
 94             unite(e.u,e.v);
 95             GG[e.u].pb({e.u,e.v,e.cost});
 96             GG[e.v].pb({e.v,e.u,e.cost});
 97         } 
 98     }
 99 }
100 
101 void dfs(int u)
102 {
103     vis[u] = 1;
104     _for(i,0,GG[u].size())
105         if(!vis[GG[u][i].v])
106         {
107             depth[GG[u][i].v] = depth[u]+1;
108             fa[GG[u][i].v][0] = u; 
109             w[GG[u][i].v][0] = GG[u][i].cost;
110             dfs(GG[u][i].v);
111         }
112 }
113 void bz()
114 {
115     _for(j,1,21)
116         _for(i,1,V+1)
117         {
118             fa[i][j] = fa[fa[i][j-1]][j-1];
119             w[i][j] = min(w[i][j-1],w[fa[i][j-1]][j-1]);
120         }
121 //    cout << w[1][1] << endl;
122 }
123 int LCA(int u,int v)
124 {
125     
126     //保证u深度较大 
127     if(depth[u]<depth[v])
128         swap(u,v);
129     int ans = w[u][0];
130     int dc = depth[u]-depth[v];
131     
132 _for(i,0,21)         if((1<<i)&dc)        {                        ans = min(ans,w[u][i]);    u=fa[u][i];    }
133     
134     if(u==v)    return ans;
135     _rep(i,20,-1)
136         if(fa[u][i]!=fa[v][i])
137         {
138             ans=min(ans,min(w[u][i],w[v][i]));
139             u = fa[u][i],v = fa[v][i];
140         }
141     int val = w[u][0];
142 //    if(fa[u][0]!=v)
143         val = min(val,w[v][0]);
144     return min(ans,val);
145 }
146 
147 int main()
148 {
149 //    memset(fa,INF,sizeof(fa));
150 //freopen("testdata.in","r+",stdin);
151     memset(w,INF,sizeof(w));
152     V = read();
153     E = read();
154     _for(i,1,E+1)
155     {
156         int a = read();
157         int b = read();
158         int c = read();
159         es.pb({a,b,c});
160     }
161     MST();
162     _for(i,1,V+1)
163         if(i==par[i])
164         {
165             dfs(i);
166         }
167     bz();
168 
169     int Q = read();
170     while(Q--)
171     {
172         int x = read();
173         int y = read();
174         if(find(x)!=find(y))
175             printf("-1\n");
176         else 
177             printf("%d\n",LCA(x,y));
178     }
179     return 0;
180 }