poj 2631 Roads in the North

最新推荐文章于 2022-03-24 19:26:51 发布

转载最新推荐文章于 2022-03-24 19:26:51 发布 · 70 阅读

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原文链接：http://www.cnblogs.com/GadyPu/p/4773395.html

本文探讨了一个经典的图论问题：寻找北境村落中两个最远村落之间的道路距离。通过构建一个图模型，利用广度优先搜索(BFS)算法两次，首先找到一个最远的节点，再从该节点出发，找到图的直径，即两个最远村落间的最大距离。

题目连接

http://poj.org/problem?id=2631

Roads in the North

Description

Building and maintaining roads among communities in the far North is an expensive business. With this in mind, the roads are build such that there is only one route from a village to a village that does not pass through some other village twice.
Given is an area in the far North comprising a number of villages and roads among them such that any village can be reached by road from any other village. Your job is to find the road distance between the two most remote villages in the area.

The area has up to 10,000 villages connected by road segments. The villages are numbered from 1.

Input

Input to the problem is a sequence of lines, each containing three positive integers: the number of a village, the number of a different village, and the length of the road segment connecting the villages in kilometers. All road segments are two-way.

Output

You are to output a single integer: the road distance between the two most remote villages in the area.

Sample Input

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

Sample Output

树的直径。。

#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<vector>
#include<queue>
#include<map>
using std::map;
using std::min;
using std::sort;
using std::pair;
using std::queue;
using std::vector;
using std::multimap;
#define pb(e) push_back(e)
#define sz(c) (int)(c).size()
#define mp(a, b) make_pair(a, b)
#define all(c) (c).begin(), (c).end()
#define iter(c) __typeof((c).begin())
#define cls(arr, val) memset(arr, val, sizeof(arr))
#define cpresent(c, e) (find(all(c), (e)) != (c).end())
#define rep(i, n) for(int i = 0; i < (int)n; i++)
#define tr(c, i) for(iter(c) i = (c).begin(); i != (c).end(); ++i)
const int N = 11000;
const int INF = 0x3f3f3f3f;
struct edge { int to, w, next; }G[N << 1];
int tot, head[N], dist[N];
void init() {
    tot = 0, cls(head, -1);
}
void add_edge(int u, int v, int w) {
    G[tot] = (edge){ v, w, head[u] }; head[u] = tot++;
    G[tot] = (edge){ u, w, head[v] }; head[v] = tot++;
}
int bfs(int u) {
    queue<int> q;
    q.push(u);
    cls(dist, -1);
    int id = 0, maxd = 0;
    dist[u] = 0;
    while(!q.empty()) {
        u = q.front(); q.pop();
        if(dist[u] > maxd) {
            maxd = dist[id = u];
        }
        for(int i = head[u]; ~i; i = G[i].next) {
            edge &e = G[i];
            if(-1 == dist[e.to]) {
                dist[e.to] = dist[u] + e.w;
                q.push(e.to);
            }
        }
    }
    return id;
}
int main() {
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w+", stdout);
#endif
    init();
    int u, v, w;
    while(~scanf("%d %d %d", &u, &v, &w)) {
        add_edge(u, v, w);
    }
    printf("%d\n", dist[bfs(bfs(1))]);
    return 0;
}

转载于:https://www.cnblogs.com/GadyPu/p/4773395.html