POJ1270 Following Orders（拓撲排序）
來源：程序員人生 發布時間：2015-09-07 08:27:00 閱讀次數：4349次
Following Orders
| Time Limit: 1000MS |
|
Memory Limit: 10000K |
| Total Submissions: 4059 |
|
Accepted: 1623 |
Description
Order is an important concept in mathematics and in computer science. For example, Zorn's Lemma states: ``a partially ordered set in which every chain has an upper bound contains a maximal element.'' Order is also important in reasoning about the fix-point
semantics of programs.
This problem involves neither Zorn's Lemma nor fix-point semantics, but does involve order.
Given a list of variable constraints of the form x < y, you are to write a program that prints all orderings of the variables that are consistent with the constraints.
For example, given the constraints x < y and x < z there are two orderings of the variables x, y, and z that are consistent with these constraints: x y z and x z y.
Input
The input consists of a sequence of constraint specifications. A specification consists of two lines: a list of variables on one line followed by
a list of contraints on the next line. A constraint is given by a pair of variables, where x y indicates that x < y.
All variables are single character, lower-case letters. There will be at least two variables, and no more than 20 variables in a specification. There will be at least one constraint, and
no more than 50 constraints in a specification. There will be at least one, and no more than 300 orderings consistent with the contraints in a specification.
Input is terminated by end-of-file.
Output
For each constraint specification, all orderings consistent with the constraints should be printed. Orderings are printed in lexicographical (alphabetical) order, one per line.
Output for different constraint specifications is separated by a blank line.
Sample Input
a b f g
a b b f
v w x y z
v y x v z v w v
Sample Output
abfg
abgf
agbf
gabf
wxzvy
wzxvy
xwzvy
xzwvy
zwxvy
zxwvy
Source
Duke Internet Programming Contest 1993,uva 124
題意：給定1串字符（互異），再給出1個字符序列，表示1種前后關系，如a b e f c d,表示a<b,e<f,c<d。
將開始給出的字符進行排序，使之符合這個關系序列。并按字典序輸出這些符合要求的字符序列。
#include<stdio.h>
#include<vector>
#include<string.h>
using namespace std;
const int N = 30;
int in[N],exist[N],mapt[N][N],path[N],n;
void topeSort(int u,int k)
{
path[k]=u;
if(k==n)
{
for(int i=1;i<=n;i++)
printf("%c",path[i]+'a');
printf("
");
return ;
}
in[u]=⑴;
for(int i=0;i<26;i++)
if(mapt[u][i])
in[i]-=mapt[u][i];
for(int i=0;i<26;i++)
if(exist[i]&&!in[i])
topeSort(i,k+1);
in[u]=0;
for(int i=0;i<26;i++)
if(mapt[u][i])
in[i]+=mapt[u][i];
}
int main()
{
int flag=0;
char str[1000];
while(gets(str))
{
if(flag)printf("
"); flag=1;
memset(in,0,sizeof(in));
memset(exist,0,sizeof(exist));
memset(mapt,0,sizeof(mapt));
n=0;
for(int i=0;str[i]!='
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