最短路径条数

求最短路径的条数

只局限于边的长度为1这种情况

P1144 最短路计数 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

初始化ans[s]=1;
求最短路时,更新ans数组
void dij()
{
	for (int i = 1; i <= n; i++)
		dis[i] = M;
	dis[1] = 0;
	priority_queue<node> q;
	q.push({ 1,0 });
	while (!q.empty())
	{
		node now = q.top(); q.pop();
		if (book[now.v] == 1)
			continue;
		book[now.v] = 1;
		for (int i = 0; i < a[now.v].size(); i++)
		{
			if (dis[a[now.v][i]] > dis[now.v] + 1)
			{
				dis[a[now.v][i]] = dis[now.v] + 1;
				ans[a[now.v][i]] = ans[now.v];
				q.push({ a[now.v][i], dis[a[now.v][i]] });
			}
			else if (dis[a[now.v][i]] == dis[now.v] + 1)
			{
				ans[a[now.v][i]] = (ans[a[now.v][i]] + ans[now.v]) % mod;
			}
		}
	}
}