Question

我希望DFS方法将路径打印为1-> 2-> 4-> 5，但它显示1-> 2-> 3-> 4-> 5可以请您提示如何使用最少量的代码附录修复该方法？

/**
 * Created by mona on 5/28/16.
 */

import java.util.Stack;

public class DepthFirstSearch {
    public static void DFS(GraphNode root, int num) {
        if (root.val == num) {
            System.out.println("root has the value "+num);
        }
        System.out.println(" current value is "+root.val);
        Stack<GraphNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            for (GraphNode g : stack.pop().neighbors) {
                if (!g.visited) {
                    System.out.println(" current value is "+g.val);
                    if (g.val == num) {
                        System.out.println("Found");
                    }
                    g.visited = true;
                    stack.push(g);
                }
            }
        }
    }

    public static void main(String[] args) {
        GraphNode n1 = new GraphNode(1);
        GraphNode n2 = new GraphNode(2);
        GraphNode n3 = new GraphNode(3);
        GraphNode n4 = new GraphNode(4);
        GraphNode n5 = new GraphNode(5);

        n1.neighbors = new GraphNode[] {n2};
        n2.neighbors = new GraphNode[] {n4,n3};
        n3.neighbors = new GraphNode[] {n4};
        n4.neighbors = new GraphNode[] {n5};
        n5.neighbors = new GraphNode[] {};

        DFS(n1, 5);
    }
}

这里是GraphNode类的代码：

/**
 * Created by mona on 5/27/16.
 */
public class GraphNode {
    int val;
    GraphNode next;
    GraphNode[] neighbors;
    boolean visited;

    GraphNode(int val) {
        this.val = val;
        this.visited = false;
    }

    GraphNode(int val, GraphNode[] neighbors) {
        this.val = val;
        this.neighbors = neighbors;
        this.visited = false;
    }

    public String toString() {
        return "value is: "+this.val;
    }

}

Answer 1

要获取节点的路径，仅仅添加您遇到的所有节点是不够的，因为您可能在图中遇到“死胡同”或添加实际上不在路径上的节点。为了防止这种情况，您需要在将节点插入堆栈时跟踪包含节点的节点：

Map<GraphNode, GraphNode> parents = new HashMap<>();
outer: while (!stack.isEmpty()) {
    GraphNode currentElement = stack.pop();
    for (GraphNode g : currentElement.neighbors) {
        if (!g.visited) {
            parents.put(g, currentElement);
            System.out.println(" current value is "+g.val);
            if (g.val == num) {
                System.out.println("Found");
                List<GraphNode> path = reconstructPath(parents, g);

                // use path, e.g.
                System.out.println(path.stream().map(n -> Integer.toString(n.val)).collect(Collectors.joining("->")));

                break outer;
            }
            g.visited = true;
            stack.push(g);
        }
    }
}

static List<GraphNode> reconstructPath(Map<GraphNode, GraphNode> parents, GraphNode end) {
     List<GraphNode> list = new ArrayList<>();
     while (end != null) {
          list.add(end);
          end = parents.get(end);
     }
     Collections.reverse(list);
     return list;
}

DFS方法无法正常工作

1 个答案: