A*算法野人传教士问题java

下面是使用A*算法解决野人传教士问题的Java代码示例：

import java.util.*;

public class AStarSearch {
    private static final int MAX_STATE_SPACE = 100000;
    private static final int MAX_PATH_COST = 100000;
    private static final int NUM_MISSIONARIES = 3;
    private static final int NUM_CANNIBALS = 3;

    // State representation of a node
    static class Node {
        int[] state = new int[2 * NUM_MISSIONARIES + 2];
        int pathCost;
        int heuristicCost;
        Node parent;

        public Node(int[] state, int pathCost, int heuristicCost, Node parent) {
            this.state = state;
            this.pathCost = pathCost;
            this.heuristicCost = heuristicCost;
            this.parent = parent;
        }

        public boolean isGoalState() {
            return state[0] == 0 && state[1] == 0;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof Node) {
                Node node = (Node) obj;
                return Arrays.equals(state, node.state);
            }
            return false;
        }

        @Override
        public int hashCode() {
            return Arrays.hashCode(state);
        }

        @Override
        public String toString() {
            return Arrays.toString(state);
        }
    }

    // Heuristic function
    private static int calculateHeuristic(Node node) {
        int numMissionariesLeft = node.state[0];
        int numCannibalsLeft = node.state[1];
        int numMissionariesRight = NUM_MISSIONARIES - numMissionariesLeft;
        int numCannibalsRight = NUM_CANNIBALS - numCannibalsLeft;

        int heuristicCost = (numMissionariesLeft + numCannibalsLeft - 2) / 2;
        if (numMissionariesLeft < numCannibalsLeft || numMissionariesRight < numCannibalsRight) {
            heuristicCost += MAX_PATH_COST;
        }
        return heuristicCost;
    }

    // Successor function
    private static List<Node> generateSuccessors(Node node) {
        List<Node> successors = new ArrayList<>();
        int[] state = node.state.clone();
        int pathCost = node.pathCost + 1;
        int numMissionariesLeft = state[0];
        int numCannibalsLeft = state[1];
        int boatPosition = state[2];
        int numMissionariesRight = NUM_MISSIONARIES - numMissionariesLeft;
        int numCannibalsRight = NUM_CANNIBALS - numCannibalsLeft;

        if (boatPosition == 0) {
            // Move one missionary to right
            if (numMissionariesLeft > 0) {
                state[0] = numMissionariesLeft - 1;
                state[2] = 1;
                state[3] = numMissionariesRight + 1;
                state[4] = numCannibalsRight;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move two missionaries to right
            if (numMissionariesLeft > 1) {
                state[0] = numMissionariesLeft - 2;
                state[2] = 1;
                state[3] = numMissionariesRight + 2;
                state[4] = numCannibalsRight;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one cannibal to right
            if (numCannibalsLeft > 0) {
                state[1] = numCannibalsLeft - 1;
                state[2] = 1;
                state[3] = numMissionariesRight;
                state[4] = numCannibalsRight + 1;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one missionary and one cannibal to right
            if (numMissionariesLeft > 0 && numCannibalsLeft > 0) {
                state[0] = numMissionariesLeft - 1;
                state[1] = numCannibalsLeft - 1;
                state[2] = 1;
                state[3] = numMissionariesRight + 1;
                state[4] = numCannibalsRight + 1;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one cannibal to right
            if (numCannibalsLeft > 1) {
                state[1] = numCannibalsLeft - 2;
                state[2] = 1;
                state[3] = numMissionariesRight;
                state[4] = numCannibalsRight + 2;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
            }
        } else {
            // Move one missionary to left
            if (numMissionariesRight > 0) {
                state[0] = numMissionariesLeft + 1;
                state[2] = 0;
                state[3] = numMissionariesRight - 1;
                state[4] = numCannibalsRight;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move two missionaries to left
            if (numMissionariesRight > 1) {
                state[0] = numMissionariesLeft + 2;
                state[2] = 0;
                state[3] = numMissionariesRight - 2;
                state[4] = numCannibalsRight;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one cannibal to left
            if (numCannibalsRight > 0) {
                state[1] = numCannibalsLeft + 1;
                state[2] = 0;
                state[3] = numMissionariesRight;
                state[4] = numCannibalsRight - 1;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one missionary and one cannibal to left
            if (numMissionariesRight > 0 && numCannibalsRight > 0) {
                state[0] = numMissionariesLeft + 1;
                state[1] = numCannibalsLeft + 1;
                state[2] = 0;
                state[3] = numMissionariesRight - 1;
                state[4] = numCannibalsRight - 1;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
                state = node.state.clone();
            }
            // Move one cannibal to left
            if (numCannibalsRight > 1) {
                state[1] = numCannibalsLeft + 2;
                state[2] = 0;
                state[3] = numMissionariesRight;
                state[4] = numCannibalsRight - 2;
                if (isValidState(state)) {
                    successors.add(new Node(state, pathCost, calculateHeuristic(new Node(state, 0, 0, null)), node));
                }
            }
        }
        return successors;
    }

    // Check if a state is valid
    private static boolean isValidState(int[] state) {
        int numMissionariesLeft = state[0];
        int numCannibalsLeft = state[1];
        int boatPosition = state[2];
        int numMissionariesRight = NUM_MISSIONARIES - numMissionariesLeft;
        int numCannibalsRight = NUM_CANNIBALS - numCannibalsLeft;

        if (numMissionariesLeft > NUM_MISSIONARIES || numMissionariesRight > NUM_MISSIONARIES ||
                numCannibalsLeft > NUM_CANNIBALS || numCannibalsRight > NUM_CANNIBALS ||
                numMissionariesLeft < 0 || numMissionariesRight < 0 || numCannibalsLeft < 0 ||
                numCannibalsRight < 0 || (numMissionariesLeft > 0 && numMissionariesLeft < numCannibalsLeft) ||
                (numMissionariesRight > 0 && numMissionariesRight < numCannibalsRight)) {
            return false;
        }
        return true;
    }

    // A* search
    public static List<Node> aStarSearch() {
        Node root = new Node(new int[]{NUM_MISSIONARIES, NUM_CANNIBALS, 0, 0, 0}, 0, 0, null);
        PriorityQueue<Node> frontier = new PriorityQueue<>(MAX_STATE_SPACE, Comparator.comparingInt(o -> o.pathCost + o.heuristicCost));
        Set<Node> explored = new HashSet<>();
        frontier.add(root);

        while (!frontier.isEmpty()) {
            Node node = frontier.poll();
            if (node.isGoalState()) {
                List<Node> path = new ArrayList<>();
                while (node != null) {
                    path.add(node);
                    node = node.parent;
                }
                Collections.reverse(path);
                return path;
            }
            explored.add(node);
            List<Node> successors = generateSuccessors(node);
            for (Node successor : successors) {
                if (!frontier.contains(successor) && !explored.contains(successor)) {
                    frontier.add(successor);
                } else if (frontier.contains(successor)) {
                    Iterator<Node> iterator = frontier.iterator();
                    while (iterator.hasNext()) {
                        Node existingNode = iterator.next();
                        if (existingNode.equals(successor) && existingNode.pathCost > successor.pathCost) {
                            iterator.remove();
                            frontier.add(successor);
                            break;
                        }
                    }
                } else if (explored.contains(successor)) {
                    Iterator<Node> iterator = explored.iterator();
                    while (iterator.hasNext()) {
                        Node existingNode = iterator.next();
                        if (existingNode.equals(successor) && existingNode.pathCost > successor.pathCost) {
                            iterator.remove();
                            frontier.add(successor);
                            break;
                        }
                    }
                }
            }
        }
        return null;
    }

    public static void main(String[] args) {
        List<Node> path = aStarSearch();
        if (path == null) {
            System.out.println("No solution found!");
        } else {
            System.out.println("Optimal solution found with cost " + path.get(path.size() - 1).pathCost);
            for (Node node : path) {
                System.out.println(Arrays.toString(node.state));
            }
        }
    }
}

在这个示例中，我们使用了一个自定义的`Node`类来表示状态空间中的一个节点。每个节点都有一个状态表示、路径成本、启发式成本和一个父节点。我们还定义了一个`calculateHeuristic`方法来计算启发式成本，一个`generateSuccessors`方法来生成一个节点的后继节点，一个`isValidState`方法来检查一个状态是否有效，以及一个`aStarSearch`方法来执行A*搜索。

在主方法中，我们调用`aStarSearch`方法来获取最优解路径，并打印出每个节点的状态表示。如果无法找到解决方案，则打印一条消息表示无法找到解决方案。