hdu 4547（LCA）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4547

思路：这题的本质还是LCA问题，但是需要注意的地方有：

1、如果Q中u,v的lca为u，那么只需一步u->...->v。

2、如果Q中u,v的lca为v，那么需abs(dist[u]  - dist[v])步。

3、否则以上情况都不满足，那么需abs(dist[v] - dist[lca(u, v)])+1步。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <map>
using namespace std;

const int MAX_N = (400000 + 20000);
struct Edge {
    int v, next;
} edge[MAX_N];

int NE, cnt, head[MAX_N], Indegree[MAX_N];
map<string, int > mp;
void Init()
{
    mp.clear();
    cnt = NE = 0;
    memset(head, -1, sizeof(head));
    memset(Indegree, 0, sizeof(Indegree));
}

void Insert(int u, int v)
{
    edge[NE].v = v;
    edge[NE].next = head[u];
    head[u] = NE++;
}

struct q_edge {
    int u, v, id, next;
} q_ee[MAX_N];

int q_ne, q_head[MAX_N];
void q_init()
{
    q_ne = 0;
    memset(q_head, -1, sizeof(q_head));
}

void q_insert(int u, int v, int id)
{
    q_ee[q_ne].u = u;
    q_ee[q_ne].v = v;
    q_ee[q_ne].id = id;
    q_ee[q_ne].next = q_head[u];
    q_head[u] = q_ne++;
}

int N, M, ans[MAX_N], dist[MAX_N];
int parent[MAX_N], lca[MAX_N];
bool vis[MAX_N];

int Find(int x)
{
    if (x == parent[x]) {
        return parent[x];
    }
    return parent[x] = Find(parent[x]);
}

void dfs(int u)
{
    parent[u] = u;
    vis[u] = true;
    for (int i = q_head[u]; ~i; i = q_ee[i].next) {
        int v = q_ee[i].v, id = q_ee[i].id;
        if (vis[v]) {

            lca[id] = Find(v);
        }
    }

    for (int i = head[u]; ~i; i = edge[i].next) {
        int v = edge[i].v;
        if (!vis[v]) {
            dist[v] = dist[u] + 1;
            dfs(v);
            parent[v] = u;
        }
    }
}

int main()
{
    int Cas;
    cin >> Cas;
    while (Cas--) {
        cin >> N >> M;

        Init();

        for (int i = 1; i < N; ++i) {
            string str1, str2;
            cin >> str1 >> str2;
            if (mp.find(str1) == mp.end()) mp[str1] = ++cnt;
            if (mp.find(str2) == mp.end()) mp[str2] = ++cnt;

            Indegree[mp[str1]]++;
            Insert(mp[str2], mp[str1]);
        }

        q_init();

        for (int i = 1; i <= M; ++i) {
            string str1, str2;
            cin >> str1 >> str2;
            q_insert(mp[str1], mp[str2], i);
            q_insert(mp[str2], mp[str1], i);
        }

        //from root;
        memset(vis, false, sizeof(vis));
        for (int i = 1; i <= cnt; ++i) {
            if (!Indegree[i]) {
                dist[i] = 0;
                dfs(i);
                break;
            }
        }

        for (int i = 0; i < q_ne; ++i) {
            if (!(i & 1)) {
                if (q_ee[i].u == q_ee[i].v) {
                    puts("0");
                } else if (q_ee[i].u == lca[q_ee[i].id]) {
                    puts("1");
                } else if (q_ee[i].v == lca[q_ee[i].id]) {
                    printf("%d\n", abs(dist[q_ee[i].v] - dist[q_ee[i].u]));
                } else {
                    printf("%d\n", abs(dist[q_ee[i].u] - dist[lca[q_ee[i].id]]) + 1);
                }
            }
        }

    }
    return 0;
}