CS61B Homework9作业回顾（附代码）

本次作业主要学会运用Disjoin Sets，DisjointSets的class作业已经写好给出。class中两个重要的方法是find和union，find(x)方法返回x的root，并在搜寻过程中递归使用path compression，使得路径上的每个node直接指向root。Union(root1，root2)方法将两个set的root指向其中等级高的那个，else使root1成为新的root。

在Maze.class中，要先创建walls[],然后使这些wall的排序处于一个随机的状态。可以将前一半的wall设置成有序，后一半的wall设置成无序的负数，然后用题目提示的交换首尾的方法使整个wall[]变成random。最后通过cell的union把不同set结合在一起，将hWalls[]或vWalls[]的boolean值设置为false，remove the walls. 

The end.

以下为题目中需要fill out 的代码：

/**
     * Fill in the rest of this method.  You should go through all the walls of
     * the maze in random order, and remove any wall whose removal will not
     * create a cycle.  Use the implementation of disjoint sets provided in the
     * set package to avoid creating any cycles.
     *
     * Note the method randInt() further below, which generates a random
     * integer.  randInt() generates different numbers every time the program
     * is run, so that you can make lots of different mazes.
     **/
    DisjointSets cell = new DisjointSets(horiz * vert);
    int numOfHWalls = horiz * (vert - 1);
    int numOfVWalls = (horiz - 1) * vert;
    int numOfWalls = numOfHWalls + numOfVWalls;
    int[] Walls = new int[numOfWalls];
    int index = 0;
    if (vert > 1){
      for (j = 0; j < vert - 1; j++){
        for (i = 0; i < horiz; i++){
          Walls[index] = j * horiz + i;
          index++;
        }
      }
    }

    if (horiz > 1){
      for (i = 0; i < horiz - 1; i++){
        for (j = 0; j < vert; j++){
          Walls[index] = -(j * horiz + i);
          index++;
        }
      }
    }

    //Order the interior walls of the maze in a random order
    int w = numOfWalls;
    while (w > 1){
      int x = randInt(w);
      int temp = Walls[x];
      Walls[x] = Walls[w-1];
      Walls[w-1] = temp;
      w--;
    }

    //Remove interior walls
    for (int k = 0; k < numOfWalls; k++){
      index = Walls[k];
      if (index > 0){
        int cell1 = cell.find(index);
        int cell2 = cell.find(index + horiz);
        if (cell1 != cell2){
          cell.union(cell1,cell2);
          hWalls[index % horiz][index/horiz] = false;
        }
      }else {
        int cell1 = cell.find(-index);
        int cell2 = cell.find(-index + 1);
        if (cell1 != cell2){
          cell.union(cell1,cell2);
          vWalls[(-index)%horiz][(-index)/horiz] = false;
        }
      }
    }