算法之A星算法（寻路）

1.启发式搜索：启发式搜索就是在状态空间中的搜索对每一个搜索的位置进行评估，得到最好的位置，再从这个位置进行搜索直到目标。这样可以省略大量无谓的搜索路径，提高了效率。在启发式搜索中，对位置的估价是十分重要的。采用了不同的估价可以有不同的效果。

　　启发算法有： 蚁群算法，遗传算法、模拟退火算法等。

2.估价算法：从当前节点移动到目标节点的预估损耗。

　　预估算法有：曼哈顿（manhattan）等。

3.算法特点：理论上时间是最优的，但空间增长是指数型的。

4.java实现：上下左右移动

 package cn.liushaofeng.algorithm;

 import java.util.ArrayList;
 import java.util.List;

 /**
  * A Star Algorithm
  * @author liushaofeng
  * @date 2015-8-24 下午11:05:48
  * @version 1.0.0
  */
 public class AstarAlgorithm
 {
     private List<Node> openList = null;
     private List<Node> closeList = null;
     private int[][] map;

     /**
      * default constructor
      * @param map data map
      */
     public AstarAlgorithm(int[][] map)
     {
         this.map = map;
         this.openList = new ArrayList<Node>();
         this.closeList = new ArrayList<Node>();
     }

     /**
      * find path
      * @param srcNode source node
      * @param desNode destination node
      * @return node path
      */
     public Node findPath(Node srcNode, Node desNode)
     {
         init(srcNode);
         do
         {
             if (openList.isEmpty())
             {
                 break;
             }

             Node node = openList.get(0);
             List<Node> aroundPoint = getAroundPoint(srcNode, node, desNode);
             openList.addAll(aroundPoint);
             closeList.add(node);
             openList.remove(node);

         } while (!findDes(desNode));

         return findNodePath(desNode);
     }

     private Node findNodePath(Node desNode)
     {
         for (Node node : openList)
         {
             if (node.getX() == desNode.getX() && node.getY() == desNode.getY())
             {
                 return node;
             }
         }
         return null;
     }

     private boolean findDes(Node desNode)
     {
         for (Node node : openList)
         {
             if (node.getX() == desNode.getX() && node.getY() == desNode.getY())
             {
                 return true;
             }
         }
         return false;
     }

     private void init(Node srcNode)
     {
         openList.add(srcNode);
     }

     // top bottom left and right, four points
     private List<Node> getAroundPoint(Node srcNode, Node nextNode, Node desNode)
     {
         int x = srcNode.getX();
         int y = srcNode.getY();

         int[] xData = new int[2];
         int[] yData = new int[2];
         if (x - 1 >= 0)
         {
             xData[0] = x - 1;
         }
         if (x + 1 < map.length)
         {
             xData[1] = x + 1;
         }

         if (y - 1 >= 0)
         {
             yData[0] = y - 1;
         }
         if (y + 1 < map[0].length)
         {
             yData[1] = y + 1;
         }

         List<Node> tmpList = new ArrayList<Node>();

         for (int i : xData)
         {
             Node node = new Node(i, y, srcNode);
             if (!isObstacle(node) && !inClosetList(node))
             {
                 calcWeight(srcNode, node, desNode);
                 tmpList.add(node);
             }
         }

         for (int i : yData)
         {
             Node node = new Node(x, i, srcNode);
             if (!isObstacle(node) && !inClosetList(node))
             {
                 calcWeight(srcNode, node, desNode);
                 tmpList.add(node);
             }
         }

         return tmpList;
     }

     private void calcWeight(Node parentNode, Node node, Node desNode)
     {
         node.setG(parentNode.getG() + 10);
         int h = Math.abs(node.getX() - desNode.getX()) + Math.abs(node.getY() - desNode.getY());
         node.setWeight(node.getG() + h * 10);
     }

     private boolean inClosetList(Node nextNode)
     {
         for (Node node : closeList)
         {
             if (node.getX() == nextNode.getX() && node.getY() == nextNode.getY())
             {
                 return true;
             }
         }
         return false;
     }

     private boolean isObstacle(Node nextNode)
     {
         return map[nextNode.getX()][nextNode.getY()] == 1;
     }

     public static void main(String[] args)
     {
         int[][] map =
         {
         { 0, 0, 0, 0, 0, 0, 0 },
         { 0, 0, 0, 0, 0, 0, 0 },
         { 0, 0, 0, 1, 0, 0, 0 },
         { 0, 0, 0, 1, 0, 0, 0 },
         { 0, 0, 0, 1, 0, 0, 0 },
         { 0, 0, 0, 0, 0, 0, 0 },
         { 0, 0, 0, 0, 0, 0, 0 } };

         AstarAlgorithm astar = new AstarAlgorithm(map);
         Node pathNode = astar.findPath(new Node(3, 1, null), new Node(3, 5, null));
         System.out.println(pathNode == null ? "Can not find path!" : pathNode.toString());
     }
 }

查看代码

数据模型

 package cn.liushaofeng.algorithm;

 import java.util.ArrayList;
 import java.util.List;

 /**
  * Node
  * @author liushaofeng
  * @date 2015-8-24 下午09:48:53
  * @version 1.0.0
  */
 public class Node
 {
     private Node parentNode;
     private int x;
     private int y;

     private int weight;
     private int g;

     /**
      * default constructor
      * @param x x point
      * @param y y point
      * @param parentNode parent node
      */
     public Node(int x, int y, Node parentNode)
     {
         this.x = x;
         this.y = y;
         this.parentNode = parentNode;
     }

     public int getG()
     {
         return g;
     }

     public void setG(int g)
     {
         this.g = g;
     }

     public Node getParentNode()
     {
         return parentNode;
     }

     public void setParentNode(Node parentNode)
     {
         this.parentNode = parentNode;
     }

     public int getX()
     {
         return x;
     }

     public void setX(int x)
     {
         this.x = x;
     }

     public int getY()
     {
         return y;
     }

     public void setY(int y)
     {
         this.y = y;
     }

     public int getWeight()
     {
         return weight;
     }

     public void setWeight(int weight)
     {
         this.weight = weight;
     }

     @Override
     public String toString()
     {
         return getPath();
     }

     private String getPath()
     {
         List<Node> dataList = new ArrayList<Node>();
         Node node = this;
         while (node != null)
         {
             dataList.add(node);
             node = node.getParentNode();
         }

         StringBuffer sb = new StringBuffer();
         for (int i = dataList.size() - 1; i >= 0; i--)
         {
             if (i == 0)
             {
                 sb.append("(" + dataList.get(i).getX() + "," + dataList.get(i).getY() + ")");
             } else
             {
                 sb.append("(" + dataList.get(i).getX() + "," + dataList.get(i).getY() + ")->");
             }
         }
         return sb.toString();
     }
 }

查看代码

 代码待调试。