P2296-寻找道路

  1 #include <bits/stdc++.h>
  2 #define _for(i,a,b) for(int i = (a);i < b;i ++)
  3 #define _rep(i,a,b) for(int i = (a);i > b;i --)
  4 #define INF 0x3f3f3f3f
  5 #define MOD 1000000007
  6 #define maxn 200003
  7 typedef long long ll;
  8 
  9 using namespace std;
 10 typedef pair<int,int> P;//first 是最短距离，second 是顶点编号
 11 inline ll read()
 12 {
 13     ll ans = 0;
 14     char ch = getchar(), last = ‘ ‘;
 15     while(!isdigit(ch)) last = ch, ch = getchar();
 16     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - ‘0‘, ch = getchar();
 17     if(last == ‘-‘) ans = -ans;
 18     return ans;
 19 }
 20 inline void write(ll x)
 21 {
 22     if(x < 0) x = -x, putchar(‘-‘);
 23     if(x >= 10) write(x / 10);
 24     putchar(x % 10 + ‘0‘);
 25 }
 26 int V,E;
 27 int st,ed;
 28 int ver[maxn],Next[maxn],head[maxn],val[maxn];
 29 int ver1[maxn],Next1[maxn],head1[maxn],val1[maxn];
 30 int novis[maxn];
 31 int cango[maxn];
 32 int vis[maxn];
 33 int tot = 0,tot1 = 0;
 34 void add(int x,int y,int w)
 35 {
 36     ver[++tot] = y,Next[tot] = head[x],head[x] = tot,val[tot] = w;
 37 }
 38 void add1(int x,int y,int w)
 39 {
 40     ver1[++tot1] = y,Next1[tot1] = head1[x],head1[x] = tot1,val1[tot1] = w;
 41 }
 42 int d[maxn];
 43 void shortest_path(int s)
 44 {
 45     priority_queue<P,vector<P>,greater<P>> que;
 46 
 47     _for(i,1,V+1)
 48     d[i] = INF;
 49     d[s] = 0;
 50     que.push(P {0,s});
 51 
 52     while(!que.empty())
 53     {
 54         P p = que.top();
 55         que.pop();
 56         int v = p.second;
 57         if(novis[v] || d[v] < p.first) continue;
 58         for(int i = head[v]; i; i = Next[i])
 59         {
 60             int y = ver[i];
 61             int w = val[i];
 62             if(d[y] > d[v] + w)
 63             {
 64                 d[y] = d[v] + w;
 65                 que.push(P {d[y],y});
 66             }
 67         }
 68     }
 69 }
 70 
 71 bool dfs(int s)
 72 {
 73     cango[s] = 1;
 74     for(int i = head1[s]; i; i = Next1[i])
 75     {
 76         int y = ver1[i];
 77         if(!vis[y])
 78         {
 79             vis[y] = 1;
 80             dfs(y);
 81         }
 82     }
 83 }
 84 bool dfs2(int s)
 85 {
 86     for(int i = head[s]; i; i = Next[i])
 87     {
 88         int y = ver[i];
 89         if(!cango[y])
 90             novis[s] = 1;
 91         if(!vis[y])
 92         {
 93             vis[y] = 1;
 94             dfs2(y);
 95         }
 96     }
 97 }
 98 int main()
 99 {
100 //    freopen("testdata.in","r+",stdin);
101     V = read(),E = read();
102     _for(i,1,E+1)
103     {
104         int a = read();
105         int b = read();
106         add(a,b,1);
107         add1(b,a,1);
108     }
109     st = read();
110     ed = read();
111     dfs(ed);
112     memset(vis,0,sizeof(vis));
113     dfs2(st);
114 /*
115     printf("\n");
116     _for(i,1,V+1)
117     printf("%d %d %d\n",i,novis[i],cango[i]);
118 */
119     
120     shortest_path(st);
121     if(d[ed]==INF)
122         printf("%d\n",-1);
123     else
124         printf("%d\n",d[ed]);
125     return 0;
126 }