> 程序如下，SPFA被注释了。
> #include <cstdio>
> #include <cstring>
> #define oo 2147483647
> #define maxn 10001
> #define maxm 31111
> 
> using namespace std;
> 
> struct edge_type
> {
>     int x,y,cost,next;
> };
> 
> int n,m,total;
> bool mark[maxn + 1];
> int stack[maxn + 1];
> int dist[maxn + 1];
> 
> int first[maxn + 1];
> edge_type g[maxm + 1];
> 
> void add(int x,int y,int cost)
> {
>     g[++total].x = x; g[total].y = y; g[total].cost = cost;
>     g[total].next = first[x];
>     first[x] = total;
> }
> 
> void spfa()
> {
>     memset(mark,0,sizeof(mark));
>     memset(dist,0,sizeof(dist));
>     for (int i=1;i<=n;i++){ //这段是BellmanFord
>         bool flag = 0;
>         for (int j=1;j<=total;j++){
>             int x = g[j].x,y = g[j].y,c = g[j].cost;
>             if (dist[x] + c>dist[y]){
>                 dist[y] = dist[x] + c;
>                 flag = 1;
>             }
>         }
>         if (!flag) break;
>     }
>     /* SPFA在这里
>     int top = 1;
>     for (top=1;top<=n + 1;top++){
>         stack[top] = top - 1;
>     }
>     while (top){
>         int x = stack[top --];
>         mark[x] = 0;
>         for (int temp=first[x];temp;temp=g[temp].next){
>             int y = g[temp].y;
>             int tmp;
>             if ((tmp = dist[x] + g[temp].cost)>dist[y]){
>                 dist[y] = tmp;
>                 if (!mark[y]){
>                     mark[y] = 1;
>                     stack[++top] = y;
>                 }
>             }
>         }
>     } */
>     /*
>     for (int i=1;i<=n;i++){
>         printf("%d ",dist[i]);
>     }
>     printf("\n");
>    */ 
> }
> 
> int main()
> {
>     freopen("1716.in","r",stdin);
>     freopen("1716.out","w",stdout);
>     while (scanf("%d",&m)==1){
>         total = 0;
>         memset(first,0,sizeof(first));
>         n = 0;
>         for (int i=1;i<=m;i++){
>             int a,b;
>             scanf("%d%d",&a,&b);
>             b ++;
>             if (a>b){
>                 int tmp = a; a = b; b = tmp;
>             }
>             if (b>n) n = b;
>             add(a,b,2);
>         }
>         for (int i=0;i<=n;i++){
>             add(i,i + 1,0);
>             add(i + 1,i,-1);
>         }
>         spfa();
>         printf("%d\n",dist[n]);
>     }
> }