URAL 1752 Tree 2 树的直径与倍增

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假设树的直径的两个端点为p0,p1。如果对于一次询问(v,k)存在点q满足要求，那么q必然在v到p0或v到p1的路径上。

剩下的就是在树上寻找p了。倍增就好了。

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <queue>
#include <cmath>
#include <stack>
#include <map>

#define LL long long
#define ULL unsigned long long
#define INF 0x3f3f3f3f

using namespace std;

const int MAXN = 20010;
const int MAXM = 15;

struct N
{
    int v,next;
}edge[2*MAXN];
int head[MAXN];
int Top;
void Link(int u,int v)
{
    edge[Top].v = v;
    edge[Top].next = head[u];
    head[u] = Top++;
}

int pa[2][MAXN][MAXM];
int dep[2][MAXN];

void InitDepFa(int s,int lay,int pre = -1,int d = 0)
{
    dep[lay][s] = d;
    pa[lay][s][0] = pre;

    for(int  p = head[s];p != -1; p = edge[p].next)
    {
        if(edge[p].v != pre)
            InitDepFa(edge[p].v,lay,s,d+1);
    }
}

void InitPa(int n,int lay)
{
    int i,j;

    for(j = 1;j < MAXM; ++j)
    {
            for(i = 1;i <= n; ++i)
            {
                if(pa[lay][i][j-1] != -1)
                    pa[lay][i][j] = pa[lay][pa[lay][i][j-1]][j-1];
                else
                    pa[lay][i][j] = -1;
            }
    }
}

int QueryKthPoint(int u,int k,int lay)
{
    for(int i = MAXM-1;i >= 0 && k; --i)
    {
        if(k >= (1<<i))
        u = pa[lay][u][i],k -= (1<<i);
    }
    return u;
}

queue<int> q;

bool mark[MAXN];

int bfs(int s)
{
    memset(mark,false,sizeof(mark));
    mark[s] = true;
    q.push(s);

    while(q.empty() == false)
    {
        s = q.front();
        q.pop();

        for(int p = head[s] ;p != -1; p = edge[p].next)
            if(mark[edge[p].v] == false)
            q.push(edge[p].v),mark[s] = true;

    }
    return s;
}

int main()
{
    int i,u,v,n,m,a,k;

    while(scanf("%d %d",&n,&m) != EOF)
    {
        memset(head,-1,sizeof(head));
        Top = 0;

        for(i = 1;i < n; ++i)
        {
            scanf("%d %d",&u,&v);
            Link(u,v);
            Link(v,u);
        }

        u = bfs(v = bfs(1));

        InitDepFa(u,0);
        InitPa(n,0);
        InitDepFa(v,1);
        InitPa(n,1);


        while(m--)
        {
            scanf("%d %d",&a,&k);
            if(dep[0][a] >=k)
                printf("%d\n",QueryKthPoint(a,k,0));
            else if(dep[1][a] >= k)
                printf("%d\n",QueryKthPoint(a,k,1));
            else
                printf("0\n");
        }
    }

    return 0;
}

URAL 1752 Tree 2 树的直径与倍增

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原文地址：http://blog.csdn.net/zmx354/article/details/45076731