Educational Codeforces Round 54 (Rated for Div. 2) D:Edge Deletion

题目链接：http://codeforces.com/contest/1076/problem/D

题意：给一个n个点,m条边的无向图。要求保留最多k条边，使得其他点到1点的最短路剩余最多。

思路：当找到单源最短路后，将其转换为一个所有点到点1都是最短路的树状结构，利用贪心确定所要保留的K条边（找离根最近的边，利用BFS）。

代码：

 #include <iostream>
 #include <cstring>
 #include <algorithm>
 #include <cmath>
 #include <cstdio>
 #include <vector>
 #include <queue>
 #include <set>
 #include <map>
 #include <stack>
 #define ll long long
 //#define local

 using namespace std;

 const int MOD = 1e9+;
 const int inf = 0x3f3f3f3f;
 const double PI = acos(-1.0);
 const int maxn = 3e5 + ;
 const int maxedge = 3e5 + ;

 struct qnode {
     int v;
     ll c;
     qnode(int v=,ll c=) : v(v), c(c) {}
     bool operator < (const qnode &r)const {//priority_queue 默认从大到小排列
         return c > r.c;
     }
 };

 struct Edge {
     int v, w, pre;
 } edge[maxedge*];

 int point[maxn]; //point[i] = -1
 int cnt;
 bool vis[maxn]; // already init in the dijkstra
 ll dist[maxn]; // already init in the dijkstra, overflow!
 vector <int> v1[maxn];
 vector <int> v2[maxn];
 struct Path {
     int u, e;
 }path[maxn];

 void AddEdge(int u, int v, int w) {
     edge[cnt].v = v;
     edge[cnt].w = w;
     edge[cnt].pre = point[u];
     point[u] = cnt++;
 }

 void Dijkstra(int n,int start) {
     memset(vis,false,sizeof(vis));
     memset(dist, 0x3f, sizeof(dist));
     priority_queue <qnode> que;
     while(!que.empty()) que.pop();
     dist[start] = ;
     que.push(qnode(start, ));
     qnode tmp;
     while(!que.empty()) {
         tmp = que.top(); que.pop();
         int u = tmp.v;
         if(vis[u]) continue;
         vis[u] = true;
         for(int i = point[u]; i != -; i = edge[i].pre) {
             int v = edge[i].v;
             int w = edge[i].w;
             if(!vis[v] && dist[v]>dist[u]+w) {
                 dist[v] = dist[u]+w;
                 que.push(qnode(v, dist[v]));
                 path[v].u = u;
                 path[v].e = i;
             }
         }
     }
 }

 void init() {
     cnt = ;
     memset(point, -, sizeof(point));
 }

 int main() {
 #ifdef local
     if(freopen("/Users/Andrew/Desktop/data.txt", "r", stdin) == NULL) printf("can't open this file!\n");
 #endif

     int n, m, k;
     scanf("%d%d%d", &n, &m, &k);
     init();
     for (int i = ; i < m; ++i) {
         int u, v, w;
         scanf("%d%d%d", &u, &v, &w);
         AddEdge(u, v, w);
         AddEdge(v, u, w);
     }
     Dijkstra(n, );
     for (int i = ; i <= n; ++i) {
         v1[path[i].u].push_back(i);
         v2[path[i].u].push_back(path[i].e);
     }
     queue<int> q;
     q.push();
     int ans = ;
     int Ans[maxn];
     while (q.size()) {
         int tmp = q.front();
         q.pop();
         for (int j = ; j < v1[tmp].size() && ans < k; ++j) {
             q.push(v1[tmp][j]);
             Ans[ans++] = v2[tmp][j]/;
         }
     }
     printf("%d\n", ans);
     for (int i = ; i < ans; ++i) {
         if (i)
             printf(" %d", Ans[i]+);
         else printf("%d", Ans[i]+);
     }
     printf("\n");
 #ifdef local
     fclose(stdin);
 #endif
     return ;
 }