TowerOfHanoi.java

package com.thealgorithms.puzzlesandgames;

import java.util.List;

/**
 * The {@code TowerOfHanoi} class provides a recursive solution to the Tower of Hanoi puzzle.
 * This puzzle involves moving a set of discs from one pole to another, following specific rules:
 * 1. Only one disc can be moved at a time.
 * 2. A disc can only be placed on top of a larger disc.
 * 3. All discs must start on one pole and end on another.
 *
 * This implementation recursively calculates the steps required to solve the puzzle and stores them
 * in a provided list.
 *
 * <p>
 * For more information about the Tower of Hanoi, see
 * <a href="https://en.wikipedia.org/wiki/Tower_of_Hanoi">Tower of Hanoi on Wikipedia</a>.
 * </p>
 *
 * The {@code shift} method takes the number of discs and the names of the poles,
 * and appends the steps required to solve the puzzle to the provided list.
 * Time Complexity: O(2^n) - Exponential time complexity due to the recursive nature of the problem.
 * Space Complexity: O(n) - Linear space complexity due to the recursion stack.
 * Wikipedia: https://en.wikipedia.org/wiki/Tower_of_Hanoi
 */
final class TowerOfHanoi {

    private TowerOfHanoi() {
    }

    /**
     * Recursively solve the Tower of Hanoi puzzle by moving discs between poles.
     *
     * @param n                The number of discs to move.
     * @param startPole        The name of the start pole from which discs are moved.
     * @param intermediatePole The name of the intermediate pole used as a temporary holding area.
     * @param endPole          The name of the end pole to which discs are moved.
     * @param result           A list to store the steps required to solve the puzzle.
     *
     *                         <p>
     *                         This method is called recursively to move n-1 discs
     *                         to the intermediate pole,
     *                         then moves the nth disc to the end pole, and finally
     *                         moves the n-1 discs from the
     *                         intermediate pole to the end pole.
     *                         </p>
     *
     *                         <p>
     *                         Time Complexity: O(2^n) - Exponential time complexity due to the recursive nature of the problem.
     *                         Space Complexity: O(n) - Linear space complexity due to the recursion stack.
     *                         </p>
     */
    public static void shift(int n, String startPole, String intermediatePole, String endPole, List<String> result) {
        if (n != 0) {
            // Move n-1 discs from startPole to intermediatePole
            shift(n - 1, startPole, endPole, intermediatePole, result);

            // Add the move of the nth disc from startPole to endPole
            result.add(String.format("Move %d from %s to %s", n, startPole, endPole));

            // Move the n-1 discs from intermediatePole to endPole
            shift(n - 1, intermediatePole, startPole, endPole, result);
        }
    }
}