ccf地铁修建

来源：互联网发布：c 矩阵运算编辑：程序博客网时间：2024/04/28 08:36

题意：无向图，从1到n的所有路径中取路径中最长的那条比较，然后输出值最小的那条。
当时想的是直接深搜，记录当前路径最大值，到n比较即可。
这样会出问题，就是从别的地方搜过来就会破坏掉原有的顺序，那么记录路径的最大值就会出现问题了。简直被自己蠢哭。

放上自己比赛时的错误代码和错误例子

#include <bits/stdc++.h>using namespace std;struct Node{    int v, val, id;    Node(){}    Node(int _v, int _val, int _id){        v = _v; val = _val; id = _id;    }};const int maxn = 1e5 + 10;const int maxm = 4e5 + 10;vector<Node> vec[maxn];int vis[maxm]; int ans = 0x3f3f3f3f; int n , m ;void dfs(int u , int value){    cout << "u  = " << u << " " << "value = " << value << endl;    system("pause");     if(u == n) {        ans = min(ans, value); return ;    }    for(int i = 0 ; i < vec[u].size(); i ++){        int v = vec[u][i].v; int val = vec[u][i].val; int id = vec[u][i].id;        if(vis[id]) continue;        vis[id] = 1;        dfs(v, max(value, val));    } }int main(){    int count = 0;    freopen("data.out", "r", stdin);    cin >> n >> m; memset(vis, 0, sizeof(vis));    for(int i = 0 ; i < m; i ++){        int a, b , c; cin >> a >> b >> c;        vec[a].push_back(Node(b, c, count ++));        vec[b].push_back(Node(a, c, count ++));    }    dfs(1, 0);    cout << ans << endl;} /*4 61 2 11 3 22 4 24 3 41 4 32 3 3*/

正解就直接spfa即可，记录每个点从1到它路径上的最大值，不断去更新，输出n点的即可。

#include <iostream>#include <algorithm>#include <vector>#include <queue>using namespace std;const int oo = 1e9;const int MAXN = 1e5 + 5;const int MAXM = 2 * 1e5 + 5;int N, M;bool inq[MAXN] = { 0 };int opt[MAXN];struct Edge{    Edge(int _from, int _to, int _weight) :from(_from), to(_to), weight(_weight) {}    int from, to;    int weight;};vector<Edge> ve;vector<int> v[MAXN];void bfs(int s){    queue<int> q;    q.push(s);    inq[s] = true;    opt[s] = 0;    for (; !q.empty();)    {        int t = q.front();        q.pop();        inq[t] = false;        for (int i = 0; i < v[t].size(); i++)        {            int e = v[t][i];            int next = ve[e].to;            int m;            if ((m = max(opt[t], ve[e].weight)) < opt[next])            {                opt[next] = m;                if (!inq[next])                {                    q.push(next);                    inq[next] = true;                }            }        }    }}int main(){//  freopen("data.out", "r", stdin);    scanf("%d%d", &N, &M);    fill(opt + 1, opt + N + 1, oo);    int f, t, w;    for (int i = 0; i < M; i++)    {        scanf("%d%d%d", &f, &t, &w);        ve.push_back(Edge(f, t, w));        ve.push_back(Edge(t, f, w));        v[f].push_back(ve.size() - 2);        v[t].push_back(ve.size() - 1);    }    bfs(1);    cout << opt[N] << endl;    return 0;}

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