Transit Tree Path 建树求最短距离

问题 D: Transit Tree Path

时间限制: 1 Sec  内存限制: 128 MB
提交: 243  解决: 76
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题目描述

You are given a tree with N vertices.
Here, a tree is a kind of graph, and more specifically, a connected undirected graph with N−1 edges, where N is the number of its vertices.
The i-th edge (1≤i≤N−1) connects Vertices ai and bi, and has a length of ci.

You are also given Q queries and an integer K. In the j-th query (1≤j≤Q):

find the length of the shortest path from Vertex xj and Vertex yj via Vertex K.
Constraints
3≤N≤105
1≤ai,bi≤N(1≤i≤N−1)
1≤ci≤109(1≤i≤N−1)
The given graph is a tree.
1≤Q≤105
1≤K≤N
1≤xj,yj≤N(1≤j≤Q)
xj≠yj(1≤j≤Q)
xj≠K,yj≠K(1≤j≤Q)

 

输入

Input is given from Standard Input in the following format:
N  
a1 b1 c1  
:  
aN−1 bN−1 cN−1
Q K
x1 y1
:  
xQ yQ

 

 

输出

Print the responses to the queries in Q lines.
In the j-th line j(1≤j≤Q), print the response to the j-th query.

 

样例输入

5
1 2 1
1 3 1
2 4 1
3 5 1
3 1
2 4
2 3
4 5

 

样例输出

3
2
4

 

提示

The shortest paths for the three queries are as follows:
Query 1: Vertex 2 → Vertex 1 → Vertex 2 → Vertex 4 : Length 1+1+1=3
Query 2: Vertex 2 → Vertex 1 → Vertex 3 : Length 1+1=2
Query 3: Vertex 4 → Vertex 2 → Vertex 1 → Vertex 3 → Vertex 5 : Length 1+1+1+1=4

 

[提交][状态]

 

给一颗树及距离，寻找经过k的路径的最短长度

以k为根建立树，用dfs

找了很长时间bug，才发现输入的数据是n-1组，不是n组

代码：

#include <bits/stdc++.h>
using namespace std;
const  int maxx=1e5+100;
const int INF=99999999;
const int MOD=1e9+7;
typedef long long ll;
int n;
struct node
{
    int r,nextt;
    ll dis;
}s[maxx*2];
ll d[maxx];
int h[maxx];
int cnt;
void add(int a,int b,ll c)
{
    s[cnt].r=b;
    s[cnt].dis=c;
    s[cnt].nextt=h[a];
    h[a]=cnt;
    cnt++;
}
void dfs(int k,int dd)
{
    for(int i=h[k]; i!=-1; i=s[i].nextt){
        int v=s[i].r;
        if(v==dd)  continue;
        d[v]=d[k]+s[i].dis;
        dfs(v,k);
    }
}
int main()
{
    scanf("%d",&n);
    int a,b;
    ll c;
    cnt=0;
    memset(h,-1,sizeof(h));
    memset(d,0,sizeof(d));
    for(int i=0; i<n-1; i++){
        scanf("%d%d%lld",&a,&b,&c);
        add(a,b,c);
        add(b,a,c);
    }
    int m,k;
    scanf("%d%d",&m,&k);
    dfs(k,-1);
    while(m--){
        scanf("%d%d",&a,&b);
        printf("%lld\n",d[a]+d[b]);
    }
    return 0;
}