HDU 5876 Sparse Graph2016 ACM/ICPC Asia Regional Dalian Online 1009

【题意】对无向图的补图求最短路

BFS：

维护一个set，里面保存还没有被访问的节点，每次bfs的时候，把与点u相连的点排除，那么剩余的点在补图里都与点u可以有一条权为1的边，访问完之后即把这些点从set里删除。也就是说扩展节点时候，只扩展与u没有边相连的点，有相边相邻的点留在外面等下一次再扩展。

【代码】

#include<stdio.h>
#include<stdlib.h>
#include<cstdio>
#include<cstring>
#include<queue>
#include<set>
#include<algorithm>
using namespace std;
const int MAXN = 2e5 + 5;
const int INF = 0x3f3f3f3f;
typedef long long LL;
int head[MAXN], e;
LL dis[MAXN];
int vis[MAXN];
struct Edge{
	int v, nxt, w;
}E[MAXN];
int N, M, S;
void init()
{
	e = 1;
	memset(head, 0, (N + 1) * sizeof(int));
	memset(vis, 0, (N + 1) * sizeof(int));
}
void add(int u, int v, int w)
{
	E[e].v = v;
	E[e].w = w;
	E[e].nxt = head[u];
	head[u] = e++;
}
struct QNode{
	int v, c;
	QNode(int _v, int _c){
		v = _v; c = _c;
	}
	bool operator < (const QNode &a) const{
		return c>a.c;
	}
};
void BFS(int st, int n)
{
	set<int>ta, tb;     //维护一个未BFS的集合，tb是temp
	queue<int>Q;
    Q.push(st);
	dis[st] = 0;
	for (int i = 1; i <= n; i++) //初始化
		if (i != st)
			ta.insert(i);
	while (!Q.empty())
	{
		int u = Q.front(); 
		Q.pop();
		for (int i = head[u]; i; i = E[i].nxt){ //把和u之间有边的点排除
			int v = E[i].v;
			if (!ta.count(v)) continue;  
			ta.erase(v); 
			tb.insert(v);
		}
		for (set<int>::iterator it = ta.begin(); it != ta.end(); it++){ //扩展
			Q.push(*it);
			dis[*it] = dis[u] + 1;
		}
		ta.swap(tb); //原ta里面除了tb里的没访问之外都访问了
		tb.clear();
	}
}

int main(void) {
	int T;
	while (scanf("%d", &T) != EOF)
	{
		int i;
		int u, v;
		for (; T; T--)
		{
			scanf("%d%d", &N, &M);
			init();
			for (i = 1; i <= M; i++)
			{
				scanf("%d%d", &u, &v);
				add(u, v, 1); 
				add(v, u, 1);
			}
			for (i = 1; i <= N; i++)
			{
				dis[i] = INF;
			}
			scanf("%d", &S);
			BFS(S, N);
			bool first = true;
			for (i = 1; i <= N; i++)
			{
				if (i == S)
					continue;
				if (false == first)
					printf(" ");
				first = false;
				if (dis[i] == INF)
					printf("-1");
				else
					printf("%d", dis[i]);
			}
			printf("\n");
		}
	}
	return 0;
}