统一成本搜索实施
我正在尝试在Udacity中观看“AI简介”课程后实施统一成本搜索。但是，我的算法没有得到正确的路径。在发布之前一直在尝试一整天。我添加了一个地图来帮助可视化场景。该算法应找到从Arad到Bucharest 
import java.util.PriorityQueue;
import java.util.HashSet;
import java.util.Set;
import java.util.Collections;
import java.util.List;
import java.util.ArrayList;
import java.util.Comparator;
//diff between uniform cost search and dijkstra algo is that UCS has a goal
public class UniformCostSearchAlgo{
public static void main(String[] args){
//initialize the graph base on the Romania map
Node n1 = new Node("Arad");
Node n2 = new Node("Zerind");
Node n3 = new Node("Oradea");
Node n4 = new Node("Sibiu");
Node n5 = new Node("Fagaras");
Node n6 = new Node("Rimnicu Vilcea");
Node n7 = new Node("Pitesti");
Node n8 = new Node("Timisoara");
Node n9 = new Node("Lugoj");
Node n10 = new Node("Mehadia");
Node n11 = new Node("Drobeta");
Node n12 = new Node("Craiova");
Node n13 = new Node("Bucharest");
Node n14 = new Node("Giurgiu");
//initialize the edges
n1.adjacencies = new Edge[]{
new Edge(n2,75),
new Edge(n4,140),
new Edge(n8,118)
};
n2.adjacencies = new Edge[]{
new Edge(n1,75),
new Edge(n3,71)
};
n3.adjacencies = new Edge[]{
new Edge(n2,71),
new Edge(n4,151)
};
n4.adjacencies = new Edge[]{
new Edge(n1,140),
new Edge(n5,99),
new Edge(n3,151),
new Edge(n6,80),
};
n5.adjacencies = new Edge[]{
new Edge(n4,99),
new Edge(n13,211)
};
n6.adjacencies = new Edge[]{
new Edge(n4,80),
new Edge(n7,97),
new Edge(n12,146)
};
n7.adjacencies = new Edge[]{
new Edge(n6,97),
new Edge(n13,101),
new Edge(n12,138)
};
n8.adjacencies = new Edge[]{
new Edge(n1,118),
new Edge(n9,111)
};
n9.adjacencies = new Edge[]{
new Edge(n8,111),
new Edge(n10,70)
};
n10.adjacencies = new Edge[]{
new Edge(n9,70),
new Edge(n11,75)
};
n11.adjacencies = new Edge[]{
new Edge(n10,75),
new Edge(n12,120)
};
n12.adjacencies = new Edge[]{
new Edge(n11,120),
new Edge(n6,146),
new Edge(n7,138)
};
n13.adjacencies = new Edge[]{
new Edge(n7,101),
new Edge(n14,90),
new Edge(n5,211)
};
n14.adjacencies = new Edge[]{
new Edge(n13,90)
};
UniformCostSearch(n1,n13);
List<Node> path = printPath(n13);
System.out.println("Path: " + path);
}
public static void UniformCostSearch(Node source, Node goal){
source.pathCost = 0;
PriorityQueue<Node> queue = new PriorityQueue<Node>(20,
new Comparator<Node>(){
//override compare method
public int compare(Node i, Node j){
if(i.pathCost > j.pathCost){
return 1;
}
else if (i.pathCost < j.pathCost){
return -1;
}
else{
return 0;
}
}
}
);
queue.add(source);
Set<Node> explored = new HashSet<Node>();
boolean found = false;
//while frontier is not empty
do{
Node current = queue.poll();
explored.add(current);
if(current.value.equals(goal.value)){
found = true;
}
for(Edge e: current.adjacencies){
Node child = e.target;
double cost = e.cost;
child.pathCost = current.pathCost + cost;
if(!explored.contains(child) && !queue.contains(child)){
child.parent = current;
queue.add(child);
System.out.println(child);
System.out.println(queue);
System.out.println();
}
else if((queue.contains(child))&&(child.pathCost>current.pathCost)){
child.parent=current;
current = child;
}
}
}while(!queue.isEmpty());
}
public static List<Node> printPath(Node target){
List<Node> path = new ArrayList<Node>();
for(Node node = target; node!=null; node = node.parent){
path.add(node);
}
Collections.reverse(path);
return path;
}
}
class Node{
public final String value;
public double pathCost;
public Edge[] adjacencies;
public Node parent;
public Node(String val){
value = val;
}
public String toString(){
return value;
}
}
class Edge{
public final double cost;
public final Node target;
public Edge(Node targetNode, double costVal){
cost = costVal;
target = targetNode;
}
}
4 个答案:
答案 0 :(得分：0)
哇。我设法搞清楚了！显然，我添加了一个双向边，而不仅仅是单向边。我没有将Edge e1从节点A转移到节点B，而是将边缘e2从节点B转移到节点A，而是移除e2，使其成为单个有向图。因此，该算法有效！
答案 1 :(得分：0)
当您找到已经在队列中的节点的新的较短路径时，您的算法的问题就出现了。您不应在current的调用之外设置poll，因为这会破坏算法方案。相反，您应该减少节点的密钥/优先级/当前最短路径成本。
我已更新您的代码以执行此操作。它提供了预期的结果。但正如我在评论中所说，当谈到渐近复杂性时，这不是最有效的解决方案。最好的选择是找到或写一个有效支持动态密钥的PriorityQueue。
但这是更新后的代码：
import java.util.PriorityQueue;
import java.util.HashSet;
import java.util.Set;
import java.util.Collections;
import java.util.List;
import java.util.ArrayList;
import java.util.Comparator;
//diff between uniform cost search and dijkstra algo is that UCS has a goal
public class UniformCostSearchAlgo{
public static void main(String[] args){
//initialize the graph base on the Romania map
Node n1 = new Node("Arad");
Node n2 = new Node("Zerind");
Node n3 = new Node("Oradea");
Node n4 = new Node("Sibiu");
Node n5 = new Node("Fagaras");
Node n6 = new Node("Rimnicu Vilcea");
Node n7 = new Node("Pitesti");
Node n8 = new Node("Timisoara");
Node n9 = new Node("Lugoj");
Node n10 = new Node("Mehadia");
Node n11 = new Node("Drobeta");
Node n12 = new Node("Craiova");
Node n13 = new Node("Bucharest");
Node n14 = new Node("Giurgiu");
//initialize the edges
n1.adjacencies = new Edge[]{
new Edge(n2,75),
new Edge(n4,140),
new Edge(n8,118)
};
n2.adjacencies = new Edge[]{
new Edge(n1,75),
new Edge(n3,71)
};
n3.adjacencies = new Edge[]{
new Edge(n2,71),
new Edge(n4,151)
};
n4.adjacencies = new Edge[]{
new Edge(n1,140),
new Edge(n5,99),
new Edge(n3,151),
new Edge(n6,80),
};
n5.adjacencies = new Edge[]{
new Edge(n4,99),
new Edge(n13,211)
};
n6.adjacencies = new Edge[]{
new Edge(n4,80),
new Edge(n7,97),
new Edge(n12,146)
};
n7.adjacencies = new Edge[]{
new Edge(n6,97),
new Edge(n13,101),
new Edge(n12,138)
};
n8.adjacencies = new Edge[]{
new Edge(n1,118),
new Edge(n9,111)
};
n9.adjacencies = new Edge[]{
new Edge(n8,111),
new Edge(n10,70)
};
n10.adjacencies = new Edge[]{
new Edge(n9,70),
new Edge(n11,75)
};
n11.adjacencies = new Edge[]{
new Edge(n10,75),
new Edge(n12,120)
};
n12.adjacencies = new Edge[]{
new Edge(n11,120),
new Edge(n6,146),
new Edge(n7,138)
};
n13.adjacencies = new Edge[]{
new Edge(n7,101),
new Edge(n14,90),
new Edge(n5,211)
};
n14.adjacencies = new Edge[]{
new Edge(n13,90)
};
UniformCostSearch(n1,n13);
List<Node> path = printPath(n13);
System.out.println("Path: " + path);
}
public static void UniformCostSearch(Node source, Node goal){
source.pathCost = 0;
PriorityQueue<Node> queue = new PriorityQueue<Node>(20,
new Comparator<Node>(){
//override compare method
public int compare(Node i, Node j){
if(i.pathCost > j.pathCost){
return 1;
}
else if (i.pathCost < j.pathCost){
return -1;
}
else{
return 0;
}
}
}
);
queue.add(source);
Set<Node> explored = new HashSet<Node>();
boolean found = false;
//while frontier is not empty
do{
Node current = queue.poll();
explored.add(current);
if(current.value.equals(goal.value)){
found = true;
}
for(Edge e: current.adjacencies){
Node child = e.target;
double cost = e.cost;
child.pathCost = current.pathCost + cost;
if(!explored.contains(child) && !queue.contains(child)){
child.parent = current;
queue.add(child);
System.out.println(child);
System.out.println(queue);
System.out.println();
}
else if((queue.contains(child))&&(child.pathCost>current.pathCost)){
child.parent=current;
// the next two calls decrease the key of the node in the queue
queue.remove(child);
queue.add(child);
}
}
}while(!queue.isEmpty());
}
public static List<Node> printPath(Node target){
List<Node> path = new ArrayList<Node>();
for(Node node = target; node!=null; node = node.parent){
path.add(node);
}
Collections.reverse(path);
return path;
}
}
class Node{
public final String value;
public double pathCost;
public Edge[] adjacencies;
public Node parent;
public Node(String val){
value = val;
}
public String toString(){
return value;
}
}
class Edge{
public final double cost;
public final Node target;
public Edge(Node targetNode, double costVal){
cost = costVal;
target = targetNode;
}
}
答案 2 :(得分：0)
这解决了添加路径的统一成本搜索问题，算法出了什么问题。
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Set;
public class UniformCostSearchAlgo {
public static void main(String[] args) {
Node n1 = new Node("Arad");
Node n2 = new Node("Zerind");
Node n3 = new Node("Oradea");
Node n4 = new Node("Sibiu");
Node n5 = new Node("Fagaras");
Node n6 = new Node("Rimnicu Vilcea");
Node n7 = new Node("Pitesti");
Node n8 = new Node("Timisoara");
Node n9 = new Node("Lugoj");
Node n10 = new Node("Mehadia");
Node n11 = new Node("Drobeta");
Node n12 = new Node("Craiova");
Node n13 = new Node("Bucharest");
Node n14 = new Node("Giurgiu");
// initialize the edges
n1.adjacencies = new Edge[] { new Edge(n2, 75), new Edge(n4, 140),
new Edge(n8, 118) };
n2.adjacencies = new Edge[] { new Edge(n1, 75), new Edge(n3, 71) };
n3.adjacencies = new Edge[] { new Edge(n2, 71), new Edge(n4, 151) };
n4.adjacencies = new Edge[] { new Edge(n1, 140), new Edge(n5, 99),
new Edge(n3, 151), new Edge(n6, 80), };
n5.adjacencies = new Edge[] { new Edge(n4, 99), new Edge(n13, 211) };
n6.adjacencies = new Edge[] { new Edge(n4, 80), new Edge(n7, 97),
new Edge(n12, 146) };
n7.adjacencies = new Edge[] { new Edge(n6, 97), new Edge(n13, 101),
new Edge(n12, 138) };
n8.adjacencies = new Edge[] { new Edge(n1, 118), new Edge(n9, 111) };
n9.adjacencies = new Edge[] { new Edge(n8, 111), new Edge(n10, 70) };
n10.adjacencies = new Edge[] { new Edge(n9, 70), new Edge(n11, 75) };
n11.adjacencies = new Edge[] { new Edge(n10, 75), new Edge(n12, 120) };
n12.adjacencies = new Edge[] { new Edge(n11, 120), new Edge(n6, 146),
new Edge(n7, 138) };
n13.adjacencies = new Edge[] { new Edge(n7, 101), new Edge(n14, 90),
new Edge(n5, 211) };
n14.adjacencies = new Edge[] { new Edge(n13, 90) };
UniformCostSearch(n1, n13);
List<Node> path = printPath(n13);
System.out.println("Path: " + path);
}
public static void UniformCostSearch(Node source, Node goal) {
List<Node> list = new ArrayList<Node>();
source.pathCost = 0;
PriorityQueue<Node> queue = new PriorityQueue<Node>(20,
new Comparator<Node>() {
// override compare method
public int compare(Node i, Node j) {
if ((i.pathCost > j.pathCost)) {
return 1;
}
else if (i.pathCost < j.pathCost) {
return -1;
}
else {
return 0;
}
}
}
);
queue.add(source);
Set<Node> explored = new HashSet<Node>();
List<Node> path = new ArrayList<Node>();
// while frontier is not empty
do {
path.clear();
Node current = queue.poll();
explored.add(current);
for (Node node = current; node != null; node = node.parent) {
path.add(node);
}
if (current.value.equals(goal.value)) {
goal.parent = current.parent;
goal.pathCost = current.pathCost;
break;
}
for (Edge e : current.adjacencies) {
Node child = e.target;
double cost = e.cost;
if ((queue.contains(child) || explored.contains(child))
&& !path.contains(child)) {
Node n = new Node(child);
list.add(n);
list.get(list.size() - 1).pathCost = current.pathCost
+ cost;
list.get(list.size() - 1).parent = current;
queue.add(list.get(list.size() - 1));
System.out.println(list.get(list.size() - 1));
System.out.println(queue);
} else if (!path.contains(child)) {
child.pathCost = current.pathCost + cost;
child.parent = current;
queue.add(child);
System.out.println(child);
System.out.println(queue);
}
}
} while (!queue.isEmpty());
}
public static List<Node> printPath(Node target) {
List<Node> path = new ArrayList<Node>();
for (Node node = target; node != null; node = node.parent) {
path.add(node);
}
Collections.reverse(path);
return path;
}
}
class Node {
public final String value;
public double pathCost;
public Edge[] adjacencies;
public Node parent;
public Node(String val) {
value = val;
}
public Node(Node node) {
int i = 0;
adjacencies = new Edge[node.adjacencies.length];
value = node.value;
pathCost = node.pathCost;
for (Edge e : node.adjacencies) {
adjacencies[i++] = e;
}
parent = node.parent;
}
public String toString() {
return value + pathCost + " ";
}
}
class Edge {
public final double cost;
public final Node target;
public Edge(Node targetNode, double costVal) {
cost = costVal;
target = targetNode;
}
}
答案 3 :(得分：0)
更改此部分代码
else if((queue.contains(child))&&(child.pathCost>current.pathCost)){
child.parent=current;
current = child;
}
到
else if((queue.contains(child))&&(child.pathCost>current.pathCost+cost)){
child.parent=current;
child.pathCost = current.pathCost+cost;
queue.remove(child);
queue.add(child);
}
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