[JSOI] 重要的城市

这样做是错的，为啥？......

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<set>
using namespace std;
const int maxn=207;
int n,m,ans;
int map[maxn][maxn];
set<int>node[maxn][maxn];
int a[maxn],top;
bool vis[maxn];
int main(){
    memset(map,0x3f3f3f3f,sizeof(map));
    cin>>n>>m; 
    for(int i=1;i<=m;i++){
        int u,v,w;cin>>u>>v>>w;
        map[u][v]=map[v][u]=w;
    }
    for(int k=1;k<=n;k++){
        for(int i=1;i<=n;i++){
            for(int j=i+1;j<=n;j++){
                if(map[i][j]>map[i][k]+map[k][j]){
                    map[i][j]=map[i][k]+map[k][j];
                    node[i][j].clear();
                    node[i][j].insert(k);
                }
                else if(map[i][j]==map[i][k]+map[k][j]&&node[i][j].count(k)) {node[i][j].clear();node[i][j].insert(k);}
                else if(map[i][j]==map[i][k]+map[k][j]&&!node[i][j].count(k)) {node[i][j].clear();}
            }
        }
    }
    for(int k=1;k<=n;k++){
        for(int i=1;i<=n;i++){
            for(int j=1;j<=n;j++){
                if(node[i][j].count(k)) a[++top]=k;
            } 
        }
    }
    sort(a+1,a+top+1);
    int sz=unique(a+1,a+top+1)-a-1;
    for(int i=1;i<=sz;i++) cout<<a[i]<<" "; 
    return 0;
}

然后最开始写的这种方法和题解一样，觉得有问题......

有一些小错误，写的不对

题解

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int maxn=207;
int n,m,ans;
int map[maxn][maxn],node[maxn][maxn];
int a[maxn],top;
bool vis[maxn],flag;
int main(){
    cin>>n>>m; 
    memset(map,0x3f3f3f,sizeof(map));
    for(int i=1;i<=n;i++) map[i][i]=0; 
    for(int i=1;i<=m;i++){
        int u,v,w;cin>>u>>v>>w;
        map[u][v]=map[v][u]=w;
    }
    for(int k=1;k<=n;k++){
        for(int i=1;i<=n;i++){
            if(k==i) continue;
            for(int j=1;j<=n;j++){
                if(k==j||j==i) continue;
                if(map[i][j]>map[i][k]+map[k][j]){
                    map[i][j]=map[i][k]+map[k][j];
                    node[i][j]=k;
                }
                else if(map[i][j]==map[i][k]+map[k][j]) {node[i][j]=-1;}
            }
        }
    }
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            if(node[i][j]!=-1) vis[node[i][j]]=true;
        }
    }
    for(int i=1;i<=n;i++)
        if(vis[i]) {cout<<i<<" ";flag=true;}
    if(!flag) cout<<"No important cities.";
    cout<<endl;
    return 0;
}