POJ 1986 Distance Queries LCA两点距离树

标题来源：POJ 1986 Distance Queries

意甲冠军：给你一棵树 q第二次查询 每次你问两个点之间的距离

思路：对于2点 u v dis(u,v) = dis(root,u) + dis(root,v) - 2*dis(roor,LCA(u,v)) 求近期公共祖先和dis数组

#include <cstdio>
#include <cstring>
#include <vector>
using namespace std;
const int maxn = 40010;
int first[maxn], head[maxn], cnt, sum;
struct edge
{
int u, v, w, next;
}e[maxn*2], qe[maxn], Q[maxn];
int ans[maxn];
int f[maxn], vis[maxn];
int d[maxn];
void AddEdge(int u, int v, int w)
{
e[cnt].u = u;
e[cnt].v = v;
e[cnt].w = w;
e[cnt].next = first[u];
first[u] = cnt++;
e[cnt].u = v;
e[cnt].v = u;
e[cnt].w = w;
e[cnt].next = first[v];
first[v] = cnt++;
}

void AddEdge2(int u, int v, int w)
{
qe[sum].u = u;
qe[sum].v = v;
qe[sum].w = w;
qe[sum].next = head[u];
head[u] = sum++;
qe[sum].u = v;
qe[sum].v = u;
qe[sum].w = w;
qe[sum].next = head[v];
head[v] = sum++;
}

int find(int x)
{
if(f[x] != x)
return f[x] = find(f[x]);
return f[x];
}
void LCA(int u, int k)
{
f[u] = u;
d[u] = k;
vis[u] = true;
for(int i = first[u]; i != -1; i = e[i].next)
{
int v = e[i].v;
if(vis[v])
continue;
LCA(v, k + e[i].w);
f[v] = u;
}
for(int i = head[u]; i != -1; i = qe[i].next)
{
int v = qe[i].v;
if(vis[v])
{
ans[qe[i].w] = find(v);
}
}
}
int main()
{
int n, m;
memset(first, -1, sizeof(first));
memset(head, -1, sizeof(head));
cnt = 0;
sum = 0;
scanf("%d %d", &n, &m);
for(int i = 0; i < m; i++)
{
int u, v, w;
char s[10];
scanf("%d %d %d %s", &u, &v, &w, s);
AddEdge(u, v, w);
}
int q;
scanf("%d", &q);
for(int i = 0; i < q; i++)
{
int u, v;
scanf("%d %d", &u, &v);
Q[i].u = u, Q[i].v = v;
AddEdge2(u, v, i);
AddEdge2(v, u, i);

}
memset(vis, 0, sizeof(vis));
d[1] = 0;
LCA(1, 0);
for(int i = 0; i < q; i++)
{
int u = Q[i].u, v = Q[i].v;
printf("%d\n", d[u] + d[v] - 2*d[ans[i]]);
}
return 0;
}