http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2733

算是一种模板题吧。。

两个发现：

1. 重复初始化queue相当费时间！最好只声明一次queue！

2. 能不用iterator就不用，这个也相当费时间！

完整代码：

/*0.732s*/

#include<bits/stdc++.h>
using namespace std;
const int mx = 1000001;

vector<int> edge[mx];
int indeg[mx], ans[mx];

int main()
{
	int n, m, i, a, b, x, v, len, Size;
	queue<int> q;
	while (scanf("%d%d", &n, &m), n)
	{
		for (i = 1; i <= n; ++i) edge[i].clear();
		memset(indeg, 0, sizeof(indeg));
		while (m--)
		{
			scanf("%d%d", &a, &b);
			edge[a].push_back(b);
			++indeg[b]; ///入度
		}
		for (i = 1; i <= n; ++i)
			if (indeg[i] == 0) q.push(i);
		len = 0;
		while (!q.empty())
		{
			ans[++len] = x = q.front();
			q.pop();
			Size = edge[x].size();
			for (i = 0; i < Size; ++i)
			{
				v = edge[x][i];
				--indeg[v];
				if (indeg[v] == 0) q.push(v);
			}
		}
		if (len == n) for (i = 1; i <= n; ++i) printf("%d\n", ans[i]);
		else puts("IMPOSSIBLE");
	}
	return 0;
}