hdu 4547(LCA+Tarjan)

解题思路：很明显的LCA问题，用Tarjan离线算法即可。这里输入的可能是字符串，所以直接用map保存。此外，根据题意，这里需要稍稍有点变化，因为cd：a\b\c...这里是一步即可完成，所以在查询a和b时，还要判断与公共祖先的关系。还要注意，这道题没有告诉根节点，所以可以根据入度为0来判断根节点。

#include<iostream>#include<string>#include<cstdio>#include<cstring>#include<map>using namespace std;const int maxn = 100005;const int WHITE = 0;const int GRAY = 1;const int BLACK = 2;struct Edge{int to,next;}edge[maxn];struct Ask{int to,next,id,lca;}ask[maxn<<1];struct Query{int a,b;}q[maxn];int n,m,cnt,cnt1,cnt2,pre[maxn],head[maxn];int fa[maxn],color[maxn],dis[maxn],in[maxn],lca[maxn];bool vis[maxn];map<string,int> mp;void init(){mp.clear();memset(pre,-1,sizeof(pre));memset(head,-1,sizeof(head));memset(in,0,sizeof(in));memset(color,0,sizeof(color));cnt = cnt1 = cnt2 = 0;}void addedge(int u,int v){edge[cnt1].to = v;edge[cnt1].next = pre[u];pre[u] = cnt1++;}void addask(int u,int v,int id){ask[cnt2].to = v;ask[cnt2].id = id;ask[cnt2].next = head[u];head[u] = cnt2++;}int find(int x){if(fa[x] == x) return x;return fa[x] = find(fa[x]);}void Tarjan(int u,int dep){fa[u] = u;dis[u] = dep;color[u] = GRAY;for(int i = pre[u]; i != -1; i = edge[i].next){int v = edge[i].to;Tarjan(v,dep+1);fa[v] = u;}color[u] = BLACK;for(int i = head[u]; i != -1; i = ask[i].next){int v = ask[i].to;if(color[u] == BLACK){int ancestor = find(v);lca[ask[i].id] = ancestor;}}}int main(){int t;string A,B;scanf("%d",&t);while(t--){scanf("%d%d",&n,&m);init();for(int i = 1; i < n; i++){cin >> A >> B;if(mp.find(A) == mp.end())mp[A] = cnt++;if(mp.find(B) == mp.end())mp[B] = cnt++;addedge(mp[B],mp[A]);in[mp[A]]++;}for(int i = 1; i <= m; i++){cin >> A >> B;addask(mp[A],mp[B],i);addask(mp[B],mp[A],i);q[i].a = mp[A], q[i].b = mp[B];}int root;for(int i = 0; i < n; i++)if(in[i] == 0){root = i;break;}Tarjan(root,0);for(int i = 1; i <= m; i++){int ancestor = lca[i];int a = q[i].a;int b = q[i].b;if(a == b)printf("0\n");else if(a == ancestor)printf("1\n");else if(b == ancestor)printf("%d\n",dis[a] - dis[b]);else printf("%d\n",dis[a] - dis[ancestor] + 1);}}return 0;}