1256: 天朝的單行道
Time Limit: 1 Sec Memory Limit: 128 MBSubmit: 169 Solved: 50
[Submit][Status][Web Board]
Description
Input
Output
Sample Input
2 1
1 2
2 1
2 1
2 0
Sample Output
0
1
-1
最短路的應用。對u->v的邊 add_edge(u,v,0);增加一條u->v反向邊 add_edge(v,u,1),求一遍1->n的最短路。
#include <iostream>
#include <queue>
#include <vector>
#include <memory.h>
using namespace std;
int N,M;
int d[5002],inq[5002];
vector<int> g[5002];
vector<int> rg[5002];
void init()
{
for(int i = 1;i <= N;i ++)
{
g[i].clear();
rg[i].clear();
}
}
void insert(int a,int b)
{
g[a].push_back(b);
rg[b].push_back(a);
}
void spfa()
{
queue<int> q;
memset(inq,0,sizeof(inq));
memset(d,0x3f,sizeof(d));
d[1] = 0;inq[1] = 1;q.push(1);
while(!q.empty())
{
int a = q.front();q.pop();inq[a] = 0;
for(int i = 0;i < g[a].size();i ++)
{
int b = g[a][i];
if(d[b] > d[a])
{
d[b] = d[a];
if(!inq[b]) inq[b] = 1,q.push(b);
}
}
for(int i = 0;i < rg[a].size();i ++)
{
int b = rg[a][i];
if(d[b] > d[a] + 1)
{
d[b] = d[a] + 1;
if(!inq[b]) inq[b] = 1,q.push(b);
}
}
}
if(d[N] < 5000) cout << d[N] << endl;
else cout << -1 << endl;
}
int main()
{
int a,b;
while(cin >> N >> M)
{
init();
for(int i = 0;i < M;i ++)
{
cin >> a >> b;
insert(a,b);
}
spfa();
}
return 0;
}