正题

题目链接：
http://poj.org/problem?id=1330

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题目大意

就是给出一棵树，求LCA（最近公共祖先）

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解题思路

用tarjan求LCA，这里给出tarjan算法

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代码

#include<cstdio>
#include<iostream>
using namespace std;
struct line{int next,to;
}a[20001];
int father[10001],n,m,q,p,v[10001],ans,tot,ls[10001],t,x,y,in[10001];
bool ok;
void addl(int x,int y)//加边
{a[++tot].to=y;a[tot].next=ls[x];ls[x]=tot;
}
int find(int x)//并查集优化
{return x==father[x]?x:find(father[x]);
}
void tarjan(int x)
{v[x]=1;for (int i=ls[x];i;i=a[i].next)//子节点{int y=a[i].to;if (v[y]) continue;tarjan(y);//tarjan一遍father[y]=x;//记录祖先}if (ok) return;//标记已找到if (v[q]==2 && x==p){printf("%d\n",find(q));//输出ok=true;return;}if (v[p]==2 && x==q)///输出{printf("%d\n",find(p));ok=true;return;}v[x]=2;
}
int main()
{scanf("%d",&t);for (;t;t--){scanf("%d",&n);tot=0;ok=0;for (int i=1;i<=n;i++){ls[i]=0;father[i]=i;v[i]=0;in[i]=0;}for (int i=1;i<n;i++){scanf("%d%d",&x,&y);in[y]++;addl(x,y);addl(y,x);}scanf("%d%d",&q,&p);for (int i=1;i<=n;i++)//寻找根if (in[i]==0) tarjan(i);//printf("%d\n",ans);}
}