UVa 10305 - Ordering Tasks (拓扑排序裸题)

John has n tasks to do. Unfortunately, the tasks are not independent and the execution of one task is only possible if other tasks have already been executed.

Input

The input will consist of several instances of the problem. Each instance begins with a line containing two integers, 1 <= n <= 100 and m. n is the number of tasks (numbered from 1 to n) and m is the number of direct precedence relations between tasks. After this, there will be m lines with two integers i and j, representing the fact that task i must be executed before task j. An instance with n = m = 0 will finish the input.

Output

For each instance, print a line with n integers representing the tasks in a possible order of execution.

Sample Input

	5 4

	1 2

	2 3

	1 3

	1 5

	0 0

Sample Output

	1 4 2 5 3

思路：
拓扑排序裸题，用的是紫书上dfs的写法

实现代码：

#include<bits/stdc++.h>
using namespace std;
const int M = 200;
int x,y,n,m,vis[M],topo[M],g[M][M],t;
bool dfs(int u){
    vis[u] = -1;  //当前在判断的
    for(int i = 1;i <= n;i ++){
        if(g[u][i]){
        if(vis[i] < 0) return false;
        if(!vis[i]&&!dfs(i)) return false;
        }
    }
    vis[u] = 1; topo[t--] = u;
    return true;
}

bool toposort(){
    t=n;
    memset(vis,0,sizeof(vis));
    for(int i = 1;i <= n;i ++)
        if(!vis[i]&&!dfs(i)) return false;
    return true;
}

int main()
{
    while(cin>>n>>m){
        if(n==0&&m==0) break;
        memset(g,0,sizeof(g));
        while(m--){
            cin>>x>>y;
            g[x][y] = 1;
        }
        if(toposort()){
            for(int i = 1;i <= n;i ++){
                if(i==n) cout<<topo[i]<<endl;
                else cout<<topo[i]<<" ";
            }
        }
    }
    return 0;
}