hello-algo/en/codes/java/chapter_divide_and_conquer/hanota.java

/**
 * File: hanota.java
 * Created Time: 2023-07-17
 * Author: krahets (krahets@163.com)
 */

package chapter_divide_and_conquer;

import java.util.*;

public class hanota {
    /* Move one disk */
    static void move(List<Integer> src, List<Integer> tar) {
        // Take a disk from the top of src
        Integer pan = src.remove(src.size() - 1);
        // Place the disk on top of tar
        tar.add(pan);
    }

    /* Solve the Tower of Hanoi problem f(i) */
    static void dfs(int i, List<Integer> src, List<Integer> buf, List<Integer> tar) {
        // If only one disk remains to be moved from src, move it directly to tar
        if (i == 1) {
            move(src, tar);
            return;
        }
        // Subproblem f(i-1): move the top i-1 disks from src to buf using tar
        dfs(i - 1, src, tar, buf);
        // Subproblem f(1): move the remaining disk of this subproblem from src to tar
        move(src, tar);
        // Subproblem f(i-1): move the top i-1 disks from buf to tar using src
        dfs(i - 1, buf, src, tar);
    }

    /* Solve the Tower of Hanoi problem */
    static void solveHanota(List<Integer> A, List<Integer> B, List<Integer> C) {
        int n = A.size();
        // Move the top n disks from A to C using B
        dfs(n, A, B, C);
    }

    public static void main(String[] args) {
        // The end of each list represents the top of the peg
        List<Integer> A = new ArrayList<>(Arrays.asList(5, 4, 3, 2, 1));
        List<Integer> B = new ArrayList<>();
        List<Integer> C = new ArrayList<>();
        System.out.println("Initial state:");
        System.out.println("A = " + A);
        System.out.println("B = " + B);
        System.out.println("C = " + C);

        solveHanota(A, B, C);

        System.out.println("After moving the disks:");
        System.out.println("A = " + A);
        System.out.println("B = " + B);
        System.out.println("C = " + C);
    }
}