解題思路:三種輸出狀況,很簡單,但用prim()寫就WA到底,感受全部狀況都考慮了啊,多是定點不必定是1到N,因此會出現bug.因此建議仍是用kruskal寫簡單一些,別怕超時,大膽寫.spa
kruskal代碼以下:
code
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 110;
const int M = 220;
const int INF = 1000000000;
int n, m, T, ans1, ans2, ise[N], f[N];
struct edge {
int u, v, cost;
}e[M];
bool cmp( edge a, edge b ) {
return a.cost < b.cost;
}
int find ( int x ) {
return f[x] == x ? x: f[x] = find(f[x]);
}
int Kru() {
int ans = 0, num = 0, id = 0;
for ( int i = 1; i <= n; ++i ) f[i] = i;
for ( int i = 0; i < m; ++i ) {
int x = e[i].u;
int y = e[i].v;
int a = find(x);
int b = find(y);
if ( a != b ) ise[id++] = i, f[a] = b, ans += e[i].cost;
}
for ( int i = 1; i <= n; ++i ) if ( i == find(i) ) num++;
if ( num > 1 ) return INF;
else return ans;
}
int Kru_1( int del ) {
int ans = 0, num = 0;
for ( int i = 1; i <= n; ++i ) f[i] = i;
for ( int i = 0; i < m; ++i ) {
if ( i == del ) continue;
int x = e[i].u;
int y = e[i].v;
int a = find(x);
int b = find(y);
if ( a != b ) f[a] = b, ans += e[i].cost;
}
for ( int i = 1; i <= n; ++i ) if ( i == find(i) ) num++;
if ( num > 1 ) return INF;
else return ans;
}
int main()
{
int idx = 1;
scanf("%d", &T);
while ( T-- ) {
scanf("%d%d", &n, &m);
for ( int i = 0; i < m; ++i )
scanf("%d%d%d", &e[i].u, &e[i].v, &e[i].cost);
sort( e, e+m, cmp );
ans1 = Kru(), ans2 = INF;
printf("Case #%d : ", idx++);
if ( ans1 == INF ) {
printf("No way\n");
continue;
}
for ( int i = 0; i < n-1; ++i ) {
int x = ise[i];
ans2 = min( ans2, Kru_1(x) );
}
if ( ans2 == INF ) printf("No second way\n");
else printf("%d\n", ans2);
}
return 0;
}
另外附上我WA的prim():get
</pre><pre name="code" class="cpp">#include<stdio.h>
#include<string.h>
#include<algorithm>
#define INF 0x3f3f3f3f
using namespace std;
struct stu{
int x,y,c;
};
stu num[210];
int n,m;
int map[110][110];
int inMST[110][110];
int MST[110][110];
int vis[110];
int per[110];
int low[110];
int see[110][110];
void inin(){
for(int i=1;i<=n;i++){
map[i][i]=0;
for(int j=1;j<i;j++){
map[i][j]=map[j][i]=INF;
}
}
}
void getMap(){
int a,b,c;
memset(see,0,sizeof(see));
for(int i=1;i<=m;i++){
scanf("%d%d%d",&a,&b,&c);
num[i].x=a;num[i].y=b;num[i].c=c;
see[a][b]++;see[b][a]++;
if(map[a][b]>c){
map[a][b]=map[b][a]=c;
}
}
}
int slove(){
int temp,v,i,j,k,fa,sum,w,x,y;
v=0;sum=INF;
memset(vis,0,sizeof(vis));
memset(inMST,0,sizeof(inMST));
memset(MST,0,sizeof(MST));
for(i=1;i<=n;i++){
low[i]=map[1][i];
per[i]=1;
}
per[1]=0;
vis[1]=1;
for(i=2;i<=n;i++){
temp=INF;
for(j=1;j<=n;j++){
if(!vis[j]&&low[j]<temp){
temp=low[j];
k=j;
}
}
if(temp==INF)return -1;
vis[k]=1;
v+=temp;
fa=per[k];
inMST[k][fa]=inMST[fa][k]=1;
for(j=1;j<=n;j++){
if(j!=k&&vis[j])
MST[k][j]=MST[j][k]=max(MST[j][fa],low[k]);
if(!vis[j]&&low[j]>map[k][j]){
low[j]=map[k][j];
per[j]=k;
}
}
}
for(i=1;i<=m;i++){
x=num[i].x;y=num[i].y;w=INF;
if(!inMST[x][y])
w=v-MST[x][y]+num[i].c;
else if(see[x][y]>1)
w=v-map[x][y]+num[i].c;
if(w<sum)sum=w;
}
return sum;
}
int main(){
int t,ans,k;
scanf("%d",&t);
k=1;
while(t--){
scanf("%d%d",&n,&m);
inin();
getMap();
ans=slove();
printf("Case #%d : ",k++);
if(ans==-1)printf("No way\n");
else if(ans==INF) printf("No second way\n");
else printf("%d\n",ans);
}
return 0;
}//錯誤案例
1
4 5
1 2 1
1 3 2
3 4 2
4 5 1
5 4 1
錯誤輸出6