【拓扑排序】CDOJ1635 琵琶弦上说相思，当时明月在，曾照彩云归

#include<cstdio>
#include<queue>
#include<cstring>
using namespace std;
int first[1010],__next[210*1010],v[210*1010],e;
void AddEdge(int U,int V){
	v[++e]=V;
	__next[e]=first[U];
	first[U]=e;
}
char a[1010][210],anss[101];
int n,len[1010],ru[1010],ans;
bool vis[1010];
priority_queue<int,vector<int>,greater<int> >Heap;
int main(){
//	freopen("d.in","r",stdin);
	scanf("%d",&n);
	for(int i=1;i<=n;++i){
		scanf("%s",a[i]+1);
		len[i]=strlen(a[i]+1);
	}
	for(int i=1;i<=200;++i){
		for(int j=1;j<n;++j){
			if(!vis[j]){
				if(a[j][i]!=a[j+1][i]){
					vis[j]=1;
					AddEdge(a[j][i],a[j+1][i]);
					++ru[a[j+1][i]];
				}
			}
		}
	}
	for(int i=‘a‘;i<=‘z‘;++i){
		if(!ru[i]){
			Heap.push(i);
		}
	}
	if(!ru[0]){
		Heap.push(0);
	}
	while(!Heap.empty()){
		int U=Heap.top(); Heap.pop();
		anss[++ans]=U;
		for(int i=first[U];i;i=__next[i]){
			--ru[v[i]];
			if(!ru[v[i]]){
				Heap.push(v[i]);
			}
		}
	}
	anss[ans+1]=‘\0‘;
	puts((ans==27 && anss[1]==0) ? anss+2 : "-1");
	return 0;
}