无向图的深度优先搜索实例

说几句题外话,我的书架总摆着几本自己认为不错的java的基础书籍,一是<<Thinking in java>>,另外两本是第二版的<<Data Structures & Algorithms in Java>>和一本国人写的<<Java面向对象程序设计教程>>.
没有什么事做的时候，自己总是会拿起这几本翻一翻，免得有一些基础的东西忘记了。

下边的例子就是<<Data Structures & Algorithms in Java>>书的一个例子,并非我所写的,这一本书我觉得是最好

的一本java数据结构的入门书籍.现在把这一个例子记录在blog上,当作温习一下啦。

进入正题,深度优等搜索有三个规则:
1、如果可能，访问一个邻接的未访问的顶点，标记它，并把放入栈中。
2、当不能执行规则1时，如果栈不空，就从栈中弹出一个顶点。
3、如果不能执行规则1和规则2，就完成了整个搜索的过程。

class StackX
   {
   private final int SIZE = 20;
   private int[] st;
   private int top;

   public StackX()           // constructor
      {
      st = new int[SIZE];    // make array
      top = -1;
      }

   public void push(int j)   // put item on stack
      { st[++top] = j; }

   public int pop()          // take item off stack
      { return st[top--]; }

   public int peek()         // peek at top of stack
      { return st[top]; }

   public boolean isEmpty()  // true if nothing on stack
      { return (top == -1); }

   }  // end class StackX

class Vertex
   {
   public char label;        // label (e.g. 'A')
   public boolean wasVisited;

   public Vertex(char lab)   // constructor
      {
      label = lab;
      wasVisited = false;
      }
   }  // end class Vertex

class Graph
   {
   private final int MAX_VERTS = 20;
   private Vertex vertexList[]; // list of vertices
   private int adjMat[][];      // adjacency matrix
   private int nVerts;          // current number of vertices
   private StackX theStack;

   public Graph()               // constructor
      {
      vertexList = new Vertex[MAX_VERTS];
                                          // adjacency matrix
      adjMat = new int[MAX_VERTS][MAX_VERTS];
      nVerts = 0;
      for(int y=0; y<MAX_VERTS; y++)      // set adjacency
         for(int x=0; x<MAX_VERTS; x++)   //    matrix to 0
            adjMat[x][y] = 0;
      theStack = new StackX();
      }  // end constructor

   public void addVertex(char lab)
      {
      vertexList[nVerts++] = new Vertex(lab);
      }

   public void addEdge(int start, int end)
      {
      adjMat[start][end] = 1;
      adjMat[end][start] = 1;
      }

   public void displayVertex(int v)
      {
      System.out.print(vertexList[v].label);
      }
// 这一块是最核心的代码
   public void dfs()  // depth-first search
      {                                 // begin at vertex 0
      vertexList[0].wasVisited = true;  // mark it
      displayVertex(0);                 // display it
      theStack.push(0);                 // push it

      while( !theStack.isEmpty() )      // until stack empty,
         {
         // get an unvisited vertex adjacent to stack top
         int v = getAdjUnvisitedVertex( theStack.peek() );
         if(v == -1)                    // if no such vertex,
            theStack.pop();
         else                           // if it exists,
            {
            vertexList[v].wasVisited = true;  // mark it
            displayVertex(v);                 // display it
            theStack.push(v);                 // push it
            }
         }  // end while

      // stack is empty, so we're done
      for(int j=0; j<nVerts; j++)          // reset flags
         vertexList[j].wasVisited = false;
      }  // end dfs

   // returns an unvisited vertex adj to v
   public int getAdjUnvisitedVertex(int v)
      {
      for(int j=0; j<nVerts; j++)
         if(adjMat[v][j]==1 && vertexList[j].wasVisited==false)
            return j;
      return -1;
      }  // end getAdjUnvisitedVertex()
   }  // end class Graph

class DFSApp
   {
   public static void main(String[] args)
      {
      Graph theGraph = new Graph();
      theGraph.addVertex('A');    // 0  (start for dfs)
      theGraph.addVertex('B');    // 1
      theGraph.addVertex('C');    // 2
      theGraph.addVertex('D');    // 3
      theGraph.addVertex('E');    // 4

      theGraph.addEdge(0, 1);     // AB
      theGraph.addEdge(1, 2);     // BC
      theGraph.addEdge(0, 3);     // AD
      theGraph.addEdge(3, 4);     // DE

      System.out.print("Visits: ");
      theGraph.dfs();             // depth-first search
      System.out.println();
      }  // end main()
   }  // end class DFSApp

输入结果:

Visits:ABCDE