Atcoder 070 D Transit Tree Path

题目链接 ：http://abc070.contest.atcoder.jp/tasks/abc070_d

题目大意：给一棵树，一个节点K和Q次询问x,y，问x倒y经过节点k的距离。

解题思路：以K为根深搜标记距离即可。

加边的时候加两条边！！！ 

数组大小开两倍！！！

爆int用ll！！！

代码：

const int inf = 0x3f3f3f3f;
const int maxn = 5e5 + 5;
struct edge{
    int to, next;
    ll val;
};
edge edges[maxn];
int tot, head[maxn], n;
ll dis[maxn];
short vis[maxn];

void init(){
    memset(head, -1, sizeof(head));
    memset(vis, 0, sizeof(vis));
    tot = 0;
}
void addEdge(int u, int v, ll w){
    edges[tot].to = v;
    edges[tot].val = w;
    edges[tot].next = head[u];
    head[u] = tot++;
}

void dfs(int u, ll val){
    vis[u] = 1; dis[u] = val;
    for(int i = head[u]; i != -1; i = edges[i].next){
        int v = edges[i].to;
        if(!vis[v]) dfs(v, val + edges[i].val);
    }
}

int main(){
    init();
    scanf("%d", &n);
    for(int i = 1; i < n; i++) {
        int u, v, w;
        scanf("%d %d %d", &u, &v, &w);
        addEdge(u, v, w);
        addEdge(v, u, w);
    }
    int q, k;
    scanf("%d %d", &q, &k);
    dfs(k, 0);
    while(q--){
        int x, y;
        scanf("%d %d", &x, &y);
        printf("%lld\n", dis[x] + dis[y]);
    }
}

题目：

D - Transit Tree Path

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Time limit : 2sec / Memory limit : 256MB

Score : 400 points

Problem Statement

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)

---

Input

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

Output

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.

---

Sample Input 1

Copy

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

Sample Output 1

Copy

3
2
4

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

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Sample Input 2

Copy

7
1 2 1
1 3 3
1 4 5
1 5 7
1 6 9
1 7 11
3 2
1 3
4 5
6 7

Sample Output 2

Copy

5
14
22

The path for each query must pass Vertex K=2.

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Sample Input 3

Copy

10
1 2 1000000000
2 3 1000000000
3 4 1000000000
4 5 1000000000
5 6 1000000000
6 7 1000000000
7 8 1000000000
8 9 1000000000
9 10 1000000000
1 1
9 10

Sample Output 3

Copy

17000000000