q9

gistfile4.js

import java.util.ArrayList;
import java.util.List;
// The Gson imports shown in the screenshot (com.google.gson.*) are not needed for this problem's core logic.

public class Player { // Assuming CoderPad uses a class named Player or similar

    /**
     * Calculates the sequence of actions to transform an atom from a starting state
     * (protonsStart, neutronsStart) to a target state (protonsTarget, neutronsTarget).
     *
     * @param protonsStart    The initial number of protons.
     * @param neutronsStart   The initial number of neutrons.
     * @param protonsTarget   The desired number of protons.
     * @param neutronsTarget  The desired number of neutrons.
     * @return A list of strings representing the actions ("PROTON", "NEUTRON", "ALPHA").
     */
    public static List<String> solve(int protonsStart, int neutronsStart, int protonsTarget, int neutronsTarget) {
        List<String> actions = new ArrayList<>();
        int currentP = protonsStart;
        int currentN = neutronsStart;

        // Loop until the target state is reached or the action limit is about to be exceeded.
        while (currentP != protonsTarget || currentN != neutronsTarget) {
            
            // Check defeat condition: List exceeds 1000 actions.
            // We check before adding an action. If size is 1000, we can't add more.
            if (actions.size() >= 1000) {
                break;
            }

            // Decision logic for choosing the next action:
            if (currentP > protonsTarget && currentN > neutronsTarget) {
                // Both protons and neutrons are too high; ALPHA helps reduce both.
                actions.add("ALPHA");
                currentP -= 2;
                currentN -= 2;
            } else if (currentP > protonsTarget) {
                // Protons are too high. ALPHA is the only way to reduce protons.
                // This will also affect neutrons, which may need correction in subsequent steps.
                actions.add("ALPHA");
                currentP -= 2;
                currentN -= 2;
            } else if (currentN > neutronsTarget) {
                // Neutrons are too high. ALPHA is the only way to reduce neutrons.
                // This will also affect protons, which may need correction in subsequent steps.
                actions.add("ALPHA");
                currentP -= 2;
                currentN -= 2;
            } else if (currentP < protonsTarget) {
                // Protons are too low. Add a proton.
                actions.add("PROTON");
                currentP++;
            } else if (currentN < neutronsTarget) {
                // At this point, currentP must be equal to protonsTarget.
                // Neutrons are too low. Add a neutron.
                actions.add("NEUTRON");
                currentN++;
            } else {
                // This 'else' block should theoretically not be reached if the loop condition
                // (currentP != protonsTarget || currentN != neutronsTarget) is true,
                // because one of the preceding conditions should have matched.
                // It acts as a safeguard against potential infinite loops not caught by the action limit.
                break; 
            }
        }
        return actions;
    }
}