227. Mock Hanoi Tower by Stacks【LintCode java】

Description

In the classic problem of Towers of Hanoi, you have 3 towers and N disks of different sizes which can slide onto any tower. The puzzle starts with disks sorted in ascending order of size from top to bottom (i.e., each disk sits on top of an even larger one). You have the following constraints:

1. Only one disk can be moved at a time.
2. A disk is slid off the top of one tower onto the next tower.
3. A disk can only be placed on the top of a larger disk.

Write a program to move the disks from the first tower to the last using stacks.

解题：本题是经典的汉诺塔问题，只要想好移动的顺序，还是挺简单的。分为三个塔：原塔（初始化n个盘子），目标塔，缓冲塔。思路是，借助目标塔（此时目标塔充当缓冲塔），先把前n-1个盘子移动到缓冲塔。然后把最后一个移动到目标塔。接着，借助原塔，把缓冲塔的盘子移动到目标塔，递归执行。代码如下：

public class Tower {
    private Stack<Integer> disks;
    /*
    * @param i: An integer from 0 to 2
    */
    public Tower(int i) {
        // create three towers
        disks = new Stack<Integer>();
    }

    /*
     * @param d: An integer
     * @return: nothing
     */
    public void add(int d) {
        // Add a disk into this tower
        if (!disks.isEmpty() && disks.peek() <= d) {
            System.out.println("Error placing disk " + d);
        } else {
            disks.push(d);
        }
    }

    /*
     * @param t: a tower
     * @return: nothing
     */
    public void moveTopTo(Tower t) {
        // Move the top disk of this tower to the top of t.
        int temp = this.getDisks().pop();
        t.add(temp);
    }

    /*
     * @param n: An integer
     * @param destination: a tower
     * @param buffer: a tower
     * @return: nothing
     */
    public void moveDisks(int n, Tower destination, Tower buffer) {
        // Move n Disks from this tower to destination by buffer tower
        if(n == 1){
            this.moveTopTo(destination);
        }else{
            this.moveDisks(n-1 , buffer, destination);
            this.moveTopTo(destination);
            buffer.moveDisks(n-1, destination, this);
        }

    }

    /*
     * @return: Disks
     */
    public Stack<Integer> getDisks() {
        // write your code here
        return disks;
    }
}

/**
 * Your Tower object will be instantiated and called as such:
 * Tower[] towers = new Tower[3];
 * for (int i = 0; i < 3; i++) towers[i] = new Tower(i);
 * for (int i = n - 1; i >= 0; i--) towers[0].add(i);
 * towers[0].moveDisks(n, towers[2], towers[1]);
 * print towers[0], towers[1], towers[2]
*/