拓扑排序

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【题目地址】POJ1094

【题目大意】题目意思比较容易理解，我就不在这里多说了。

【题目分析】该题需要利用拓扑排序进行处理

【代码】

#include <iostream>#include <cstdio>#include <cstring>#include <cstdlib>#include <stack>#define mem(x)  memset((x),0,sizeof((x)))const int N=30;using namespace std;int graph[N][N];int indegree[N];int list[N];int toposort(int n) {    int in[N];    memcpy(in,indegree,sizeof(indegree)); //复制入度数组，以免对主函数中的indegree有影响    stack<int> s;    int i;    for(i=0; i<n; i++)        if(!in[i])            s.push(i);//所有入度为0的点入栈，如果这些点多于1的话，序列不确定    int flag=0;    int t,j=0;    while(!s.empty()) {        if(s.size()>1)            flag=1;    //序列不确定        t=s.top();        s.pop();        list[j++]=t;   //记录出栈的数字        for(i=0; i<n; i++)            if(graph[t][i])                if(--in[i]==0)                    s.push(i);//入度为0的点入栈    }    if(j!=n)//不能拓扑排序，即有环        return 1;    else if(flag==1)//有多种排序方式，不能唯一确定        return 2;    return 0;//序列能够被唯一确定}int main(){    freopen("in.txt","r",stdin);    //freopen("out.txt","w",stdout);    int n,m;    int determined,Inconsistency;    int res;    char a,b;    while(scanf("%d%d",&n,&m) != EOF && n+m){        getchar();        mem(graph);        mem(indegree);        determined = 0;        Inconsistency = 0;        for(int i = 1; i <= m; i++){            scanf("%c<%c",&a,&b);            getchar();            if(!determined && !Inconsistency){                if(graph[b-'A'][a-'A'] == 1){                    Inconsistency = 1;                    printf("Inconsistency found after %d relations.\n",i);                    continue;                }                if(graph[a-'A'][b-'A'] == 0){                    graph[a-'A'][b-'A'] = 1;                    indegree[b-'A']++;                }                res = toposort(n);                if(res == 0){                    printf("Sorted sequence determined after %d relations: ",i);                    for(int j = 0; j < n; j++){                        printf("%c",list[j] + 'A');                    }                    printf(".\n");                    determined = 1;                }                else if(res == 1){                    printf("Inconsistency found after %d relations.\n",i);                    Inconsistency = 1;                }            }        }        if(!determined && !Inconsistency){            printf("Sorted sequence cannot be determined.\n");        }    }    return 0;}

【注】以上代码参考了博主 pushing my way 关于该题的解题代码

【后记】toposort函数我还是没怎么看懂

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