【POJ 1094】拓扑排序

题意

给出n，代表有以A开始的n个字母，给出它们的m个小于关系（A<B）。
如果前i个关系可以确定n个字母的一个顺序就输出：

Sorted sequence determined after i relations: 排好的字母.

如果前i个关系开始导致矛盾，就输出：

Inconsistency found after i relations.

m个关系后还不能确定顺序就输出：

Sorted sequence cannot be determined.　　　　

代码

/*
g[i][j]为邻接矩阵，e[i]为i的入度
in==1代表有矛盾，d==1代表确定顺序
so存排好的顺序
*/
#include<cstdio>
#include<cstring>
#define N 30
int n, m, g[N][N], e[N], c[N], a, b, in, d, so[N];
char va, vb;
void topo()
{
    memcpy(c, e, sizeof e);//复制e到c，因为后面要对c改动，所以不能直接用e
    int i, top = -1;
    for(i = 0; i < n; i++)
        if(!c[i])
        {
            c[i] = top;//模拟下标堆栈，把入度为0的点通过下标堆栈到c中
            top = i;
        }
    for(i = 0; i < n; i++)
    {
        if(top == -1)//n次循环中，若某次找不到入度为0的点，就会回到-1
        {
            in = 1;//说明存在环
            return;
        }
        int u = top;//以top为起点
        top = c[u];
        so[i] = u;
        for(int v = 0; v < n; v++)
            if(g[u][v] && (--c[v]) == 0)//去掉u出发的边后，入度为0的点
            {
                c[v] = top; //堆栈
                top = v;
            }
    }
    d = 1;
    for(i = 1; i < n; i++)//判断是否是确定的顺序
        if(!g[so[i - 1]][so[i]])d = 0;//如果出现相邻两个没有给过偏序关系，那么就是不确定的顺序
 
}
int main()
{
    //freopen("in.txt", "r", stdin);
    while(~scanf("%d%d ", &n, &m) && n)
    {
        memset(g, 0, sizeof g);
        memset(e, 0, sizeof e);
        d = in = 0;
        for(int i = 1; i <= m; i++)
        {
            scanf("%c<%c ", &va, &vb);
            a = va - 'A';
            b = vb - 'A';
            if(!g[a][b] && !d && !in)
            {
                g[a][b] = 1;
                e[b]++;
                topo();
                if(in)
                    printf("Inconsistency found after %d relations.
", i);
                else if(d)
                {
                    printf("Sorted sequence determined after %d relations: ", i);
                    for(int j = 0; j < n; j++)printf("%c", so[j] + 'A');
                    printf(".
");
                }
            }
        }
        if(!d && !in)printf("Sorted sequence cannot be determined.
");
    }
}
 

　　这个是我参考lrj的《入门经典》的板子写的，耗时0ms

#include <cstdio>
#include <cstring>
#define N 30
int n, m, g[N][N], c[N], t, ans;
char a, b, topo[N];
int dfs(int u)
{
    c[u] = -1;
    for(int v = 0; v < n; v++)if(g[u][v])
        {
            if(c[v] < 0)return 0;
            else if(!c[v] && !dfs(v))return 0;
        }
    c[u] = 1;
    topo[--t] = u;
    return 1;
}
int toposort()
{
    t = n;
    memset(c, 0, sizeof c);
    for(int u = 0; u < n; u++)
        if(!c[u] && !dfs(u))return 0;
    for(int i = 1; i < n; i++)
        if(!g[topo[i - 1]][topo[i]])return -1;
    return 1;
}
int main()
{
 //  freopen("in.txt", "r", stdin);
    while(~scanf("%d%d ", &n, &m) && n)
    {
        ans = -1;
        memset(g, 0, sizeof g);
        for(int i = 1; i <= m; i++)
        {
            scanf("%c<%c ", &a, &b);
            g[a - 'A'][b - 'A'] = 1;
            if(ans == -1)
            {
                ans = toposort();
                if(!ans)
                    printf("Inconsistency found after %d relations.
", i);
                else if(ans == 1)
                {
                    printf("Sorted sequence determined after %d relations: ", i);
                    for(int j = 0; j < n; j++)printf("%c", topo[j] + 'A');
                    printf(".
");
                }
            }
        }
        if(ans == -1)printf("Sorted sequence cannot be determined.
");
    }
    return 0;
}