poj1986 Distance Queries

Posted 2020-08-12

http://poj.org/problem?id=1986 (题目链接)

题意

　　给出一棵树，询问树上两点间距离

Solution

　　lca。

代码

// poj1985
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<cmath>
#define LL long long
#define MOD 100000000
#define inf 2147483640
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std;

const int maxn=400010;
struct edge {int to,next,w;}e[maxn<<1];
int n,m,cnt,head[maxn],deep[maxn],sum[maxn],bin[40],fa[maxn][40];

void link(int u,int v,int w) {
	e[++cnt].to=v;e[cnt].next=head[u];head[u]=cnt;e[cnt].w=w;
	e[++cnt].to=u;e[cnt].next=head[v];head[v]=cnt;e[cnt].w=w;
}
void dfs(int x) {
	for (int i=1;i<=30;i++) fa[x][i]=fa[fa[x][i-1]][i-1];
	for (int i=head[x];i;i=e[i].next) if (e[i].to!=fa[x][0]) {
			deep[e[i].to]=deep[x]+1;
			sum[e[i].to]=sum[x]+e[i].w;
			fa[e[i].to][0]=x;
			dfs(e[i].to);
		}
}
int lca(int x,int y) {
	if (deep[x]<deep[y]) swap(x,y);
	int t=deep[x]-deep[y];
	for (int i=0;bin[i]<=t;i++) if (bin[i]&t) x=fa[x][i];
	for (int i=30;i>=0;i--) if (fa[x][i]!=fa[y][i]) x=fa[x][i],y=fa[y][i];
	return x==y ? x : fa[x][0];
}
int main() {
	bin[0]=1;for (int i=1;i<=30;i++) bin[i]=bin[i-1]<<1;
	scanf("%d%d",&n,&m);
	char ch;
	for (int u,v,w,i=1;i<=m;i++) {
		scanf("%d%d%d %c",&u,&v,&w,&ch);
		if (u==v) continue;
		link(u,v,w);
	}
	dfs(1);
	int q;scanf("%d",&q);
	while (q--) {
		int x,y;
		scanf("%d%d",&x,&y);
		int f=lca(x,y);
		printf("%d\n",sum[x]+sum[y]-2*sum[f]);
	}
	return 0;
}