我正在尝试用Java实现深度优先搜索算法。知道为什么这个方法会进入一个无限循环吗?
public static void searchmap(map Romania)
{
Problem?-----> citylist.push(current.getcity(nextCity));
正如您在CMD中看到的,当Drobeta没有添加到已访问城市的ArrayList中时,就会发生无限循环。有什么想法为什么不添加这个吗?
Step 1, In Arad, Distance 140
Zerind
[Arad]
Timisoara
[Arad]
Step 2, In Timisoara, Distance 258
Lugoj
[Arad, Timisoara]
Step 3, In Lugoj, Distance 369
Mehadia
[Arad, Timisoara, Lugoj]
Step 4, In Mehadia, Distance 444
Lugoj
[Arad, Timisoara, Lugoj, Mehadia]
Timisoara
[Arad, Timisoara, Lugoj, Mehadia]
Mehadia
[Arad, Timisoara, Lugoj, Mehadia]
**Drobeta**
[Arad, Timisoara, Lugoj, Mehadia]
Lugoj
[Arad, Timisoara, Lugoj, Mehadia]
Timisoara
[Arad, Timisoara, Lugoj, Mehadia]
Mehadia
[Arad, Timisoara, Lugoj, Mehadia]
**Drobeta**
[Arad, Timisoara, Lugoj, Mehadia]
Lugoj
直到第4步,一切都做得很好。
节点图如下所示:罗马尼亚树
这些是我的课程:
import java.util.ArrayList;
public class city
{
private String name;
private int numconnections = 0;
private ArrayList nextcity = new ArrayList();
private ArrayList distance = new ArrayList();
// Straight line distance to Bucharest
private int SLD;
// Hacks to make searching meaningful and illustrative. See source.
public int depth;
public city camefrom;
public boolean visited;
public city (String n, int s)
{
this.name = n;
this.SLD = s;
}
@SuppressWarnings ("unchecked")
public void addconnection (city conn, int dist)
{
numconnections++;
nextcity.add (conn);
distance.add (dist);
}
public int getSLD()
{
return this.SLD;
}
public int getconnections()
{
return numconnections;
}
public city getcity (int index)
{
return (city)nextcity.get(index);
}
public int getdist (int index)
{
return (int)distance.get(index);
}
public String getname()
{
return this.name;
}
}
public class map
{
city Oradea = new city ("Oradea", 380);
city Zerind = new city ("Zerind", 374);
city Arad = new city ("Arad", 366);
city Timisoara = new city ("Timisoara", 329);
city Lugoj = new city ("Lugoj", 244);
city Mehadia = new city ("Mehadia", 241);
city Drobeta = new city ("Drobeta", 242);
city Craiova = new city ("Craiova", 160);
city Rimnicu = new city ("Rimnicu", 193);
city Sibiu = new city ("Sibiu", 253);
city Pitesi = new city ("Pitesi", 100);
city Fagaras = new city ("Fagaras", 176);
city Bucharest = new city ("Bucharest", 0);
city Giurgiu = new city ("Giurgiu", 77);
city Hirsova = new city ("Hirsova", 151);
city Eforie = new city ("Eforie", 161);
city Urziceni = new city ("Urziceni", 80);
city Vaslui = new city ("Vaslui", 199);
city Iasi = new city ("Iasi", 226);
city Neamt = new city ("Neamt", 234);
public map ()
{
Oradea.addconnection (Zerind, 71);
Oradea.addconnection (Sibiu, 151);
Zerind.addconnection (Oradea, 71);
Zerind.addconnection (Arad, 75);
Arad.addconnection (Sibiu, 140);
Arad.addconnection (Zerind, 75);
Arad.addconnection (Timisoara, 118);
Timisoara.addconnection (Arad, 118);
Timisoara.addconnection (Lugoj, 111);
Lugoj.addconnection (Timisoara, 111);
Lugoj.addconnection (Mehadia, 70);
Mehadia.addconnection (Drobeta, 75);
Mehadia.addconnection (Lugoj, 70);
Drobeta.addconnection (Mehadia, 75);
Drobeta.addconnection (Craiova, 120);
Craiova.addconnection (Drobeta, 120);
Craiova.addconnection (Rimnicu, 146);
Craiova.addconnection (Pitesi, 120);
Rimnicu.addconnection (Craiova, 146);
Rimnicu.addconnection (Sibiu, 80);
Rimnicu.addconnection (Pitesi, 97);
Sibiu.addconnection (Arad, 140);
Sibiu.addconnection (Oradea, 151);
Sibiu.addconnection (Fagaras, 99);
Sibiu.addconnection (Rimnicu, 80);
Pitesi.addconnection (Craiova, 120);
Pitesi.addconnection (Rimnicu, 97);
Pitesi.addconnection (Bucharest, 101);
Fagaras.addconnection (Sibiu, 99);
Fagaras.addconnection (Bucharest, 211);
Bucharest.addconnection (Fagaras, 211);
Bucharest.addconnection (Pitesi, 101);
Bucharest.addconnection (Giurgiu, 90);
Bucharest.addconnection (Urziceni, 85);
Giurgiu.addconnection (Bucharest, 90);
Urziceni.addconnection (Hirsova, 98);
Urziceni.addconnection (Bucharest, 85);
Urziceni.addconnection (Vaslui, 142);
Hirsova.addconnection (Eforie, 86);
Hirsova.addconnection (Urziceni, 98);
Eforie.addconnection (Hirsova, 86);
Vaslui.addconnection (Urziceni, 142);
Vaslui.addconnection (Iasi, 92);
Iasi.addconnection (Vaslui, 92);
Iasi.addconnection (Neamt, 87);
Neamt.addconnection (Iasi, 87);
}
}
很抱歉,你做错了太多。
-
看看你的地图。这些数字是从哪里来的?140是到锡比乌的距离。但你不是要去蒂米索拉吗?如果你从阿拉德开始,而你的访问城市列表中只包含阿拉德,为什么会有任何距离?看看你的输出和地图,看看它是否有意义。当然,直到第4步,它才开始工作。
-
如果你到达布加勒斯特,你将如何检测?它不在for循环中,这是你推动和弹出城市的地方。也许先检查一下布加勒斯特是否在连接列表中。
-
你打算怎么回到正轨?您不会在任何地方从访问列表中弹出城市。如果你没有到达布加勒斯特就走到了死胡同,会发生什么?
-
无限循环的产生是因为你在for循环的前半部分推送一个城市一次,然后在while循环中弹出它,但你的算法是错误的,所以你永远不会进步。它达到了一个循环,因为在这一点上,你只会在循环中推动一个城市,然后弹出它,再做一次。
要弄清楚你做错了什么,请在for循环和while循环的末尾打印出两个城市列表(visited和cityList),确保你可以从每一行的打印位置分辨出来。它们不是你想象的那样。
由于这是深度优先搜索,也许您应该考虑使用递归函数。
确保在for循环运行时numconnections
的值不会上升。这可能是