hdu 4912 Paths on the tree(lca+贪心)

题意：给出一颗树和m条路径，要求选择尽量多的路径，并且这些路径不相交。

思路：没想到这题是贪心，比较忧伤。首先求一下每对路径的lca，按照lca的层数排序，在深一层的优先级高。那么就可以贪心了，每次选择层数最深的那一个，如果两个端点没有被标记，那么就选择这条路径，把lca的子树都标记掉。

代码：

#pragma comment(linker, "/STACK:1024000000,1024000000")#include<iostream>#include<cstdio>#include<cstring>#include<string>#include<algorithm>#include<map>#include<queue>#include<stack>#include<set>#include<cmath>#include<vector>#define inf 0x3f3f3f3f#define Inf 0x3FFFFFFFFFFFFFFFLL#define eps 1e-9#define pi acos(-1.0)using namespace std;typedef long long ll;const int maxn=100000+10;struct Edge{    int v,next;    Edge(int v=0,int next=0):v(v),next(next){}}edges[maxn<<1];struct Node{    int u,v,id;    Node(int u=0,int v=0,int id=0):u(u),v(v),id(id) {}}node[maxn];int head[maxn],nEdge;int LCA[maxn],pa[maxn],d[maxn];bool vis[maxn];vector<pair<int,int> >querys[maxn];int Find(int x){    return x==pa[x]?x:pa[x]=Find(pa[x]);}void AddEdges(int u,int v){    edges[++nEdge]=Edge(v,head[u]);    head[u]=nEdge;    edges[++nEdge]=Edge(u,head[v]);    head[v]=nEdge;}void dfs(int u,int fa){    pa[u]=u;    pair<int,int>pii;    for(int i=0;i<(int)querys[u].size();++i)    {        pii=querys[u][i];        if(d[pii.first]==-1) continue;        LCA[pii.second]=Find(pii.first);    }    for(int k=head[u];k!=-1;k=edges[k].next)    {        int v=edges[k].v;        if(v==fa) continue;        d[v]=d[u]+1;        dfs(v,u);        pa[v]=u;    }}bool cmp(Node a,Node b){    int u=LCA[a.id];    int v=LCA[b.id];    return d[u]>d[v];}void Init(int n){    memset(vis,0,sizeof(vis));    memset(head,0xff,sizeof(head));    nEdge=-1;    memset(d,0xff,sizeof(d));    for(int i=0;i<=n;++i)        querys[i].clear();}void tour(int u){    vis[u]=true;    for(int k=head[u];k!=-1;k=edges[k].next)    {        int v=edges[k].v;        if(vis[v]||d[v]<d[u]) continue;        tour(v);    }}int main(){//    freopen("in.txt","r",stdin);//    freopen("out.txt","w",stdout);    int n,m;    while(~scanf("%d%d",&n,&m))    {        Init(n);        int u,v;        for(int i=1;i<n;++i)        {            scanf("%d%d",&u,&v);            AddEdges(u,v);        }        for(int i=0;i<m;++i)        {            scanf("%d%d",&u,&v);            querys[u].push_back(make_pair(v,i));            querys[v].push_back(make_pair(u,i));            node[i].u=u;node[i].v=v;            node[i].id=i;        }        d[1]=0;        dfs(1,-1);        sort(node,node+m,cmp);        int cnt=0;        for(int i=0;i<m;++i)        {            if(!vis[node[i].u]&&!vis[node[i].v])            {                cnt++;                tour(LCA[node[i].id]);            }        }        printf("%d\n",cnt);    }    return 0;}