poj 1094 Sorting It All Out 拓扑排序

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题目：http://poj.org/problem?id=1094

题意：给定n个字母，和m个它们之间的偏序关系，判断是否可以排成一个全序序列，在第几条关系后能得出全序序列，或者得出有矛盾。否则最后输出无法判断

思路：拓扑排序，如果有环肯定无法排成全序序列。进行拓扑排序时，每次只能有一个点入度为0，否则无法判断。

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <vector>#include <map>#include <cmath>#include <string>#include <sstream>#include <cctype>#include <queue>using namespace std;typedef long long ll;const int N = 30;vector<int> G[N];int n, m, k;int degree[N], tmp[N], res[N];bool used[N];int toposort(){    queue<int> que;    memcpy(tmp, degree, sizeof degree);    for(int i = 0; i < n; i++)        if(tmp[i] == 0)            que.push(i);    k = 0;    bool f = false;    while(! que.empty())    {        int v = que.front(); que.pop();        res[k++] = v;        if(! que.empty()) f = true; //入度为0的点不止一个，无法排出全序序列        for(int i = 0; i < G[v].size(); i++)        {            if(--tmp[G[v][i]] == 0)                que.push(G[v][i]);        }    }    if(k != n) return -1;    if(f) return 0;    else return 1;}int main(){    char a, b;    while(scanf("%d%d", &n, &m), n || m)    {        memset(degree, 0, sizeof degree);        bool found = false;        for(int i = 0; i < m; i++)        {            scanf(" %c<%c", &a, &b);            if(found) continue;            int x = a - 'A', y = b - 'A';            degree[y]++;            G[x].push_back(y);            int r = toposort();            if(r == 1)            {                printf("Sorted sequence determined after %d relations: ", i + 1);                for(int i = 0; i < k; i++)                    printf("%c", res[i] + 'A');                printf(".\n");                found = true;            }            else if(r == -1)            {                printf("Inconsistency found after %d relations.\n", i + 1);                found = true;            }        }        if(!found) printf("Sorted sequence cannot be determined.\n");        for(int i = 0; i < 26; i++)            G[i].clear();    }    return 0;}

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