java字符串全排列问题

import java.util.Iterator;
import java.util.TreeSet;

public class Sort {

     private String[] b = new String[] {"1","2","2","3","4","6"};

     private int n = b.length;

     private boolean[] visited = new boolean[n];

     private int[][] a = new int[n][n];

     private String result = "";

     private TreeSet<String> set = new TreeSet<String>();

     public static void main(String[] args) {
         new Sort().start();
     }

     private void start() {

         // Initial the map a[][]

         for (int i = 0; i < n; i++) {
             for (int j = 0; j < n; j++) {
                 if (i == j) {
                     a[i][j] = 0;
                 } else {
                     a[i][j] = 1;
                 }
             }
         }

         // 3 and 5 can not be the neighbor.

         a[3][5] = 0;
         a[5][3] = 0;

         // Begin to depth search.

         for (int i = 0; i < n; i++) {
             this.depthFirstSearch(i);
         }

         // Print result treeset.

         Iterator it = set.iterator();
         while (it.hasNext()) {
             String string = (String) it.next();
             System.out.println(string);
         }
     }

     private void depthFirstSearch(int startIndex) {
         visited[startIndex] = true;
         result = result + b[startIndex];
         if (result.length() == n) {
             // "4" can not be the third position.

             if (result.indexOf(" 4 ") != 2) {
                 // Filt the duplicate value.

                 set.add(result);
             }
         }
         for (int j = 0; j < n; j++) {
             if (a[startIndex][j] == 1 && visited[j] == false) {
                 depthFirstSearch(j);
             }
         }

         // restore the result value and visited value after listing a node.

         result = result.substring(0, result.length() - 1);
         visited[startIndex] = false;
     }
}