使用DFS查找一组字符串

Question

We have a self-defined data structure have the following attributes: id, a list of id it has access to(can be empty), string(can be empty)

eg

“Thomas” [“Donald”, “Barbara”] “a”
“Donald” [“Sarah”, “Lisa”] “”
“Lisa” [] “c”
“Sarah” [“Lisa”, “Thomas”, “Barbara”] “d”
“Barbara” [“Sarah”] “e”

Given a start id “Donald” , and target string set we are trying to collect is [“a”, “c”, “d”] , list all possible ways to visit all strings in target set. Every time we try to access an item through id, we need to make sure this id is valid. For the example given above, one possible path is [[“Donald”], [“Sarah”, “d”], [“Thomas”, “a”], [“Donald”], [“Lisa”, “c”]] .

Another example, Given a start id "Sarah" , and target string set we are trying to collect is [“a”, “d”], one possible path is [["Sarah", "d"], ["Thomas", "a"]] .

Here is how I try to address this problem. Since the database could be very large, I store all the variable as in HashMap as a class variable. Then use DFS to search for the list of target strings.

Every time it searches the current item and also all the items in idList, if a match is found add this item to path, until it finds all string in targets. The process is done recursively.

However, this isn't a typical DFS, cause the list it's trying to traverse is different every time, also, the first item should always be added to path no matter what. That's why I get stuck.

I made a careless mistake in the code, thanks @Mil4n for pointing it out. Here is modified code:

import java.util.*;


public class TestFinder {
    Map<String, Item> map = new HashMap<String, Item>();


    public void finder(List<String> targets, String startid){
        List<List<String>> path = new ArrayList<List<String>>();
        List<List<List<String>>> result = new ArrayList<List<List<String>>>();

        dfs(startid, targets, path, result);

        // print out result
        System.out.println(result.size());
        for(int i=0; i<result.size(); i++){
            for(int j=0; j<result.get(i).size(); j++){
                for(int k=0; k<result.get(i).get(j).size(); k++){
                    System.out.println(result.get(i).get(j).get(k) + " ");
                }

            }
            System.out.println();
        }
    }

    public void dfs(String id, List<String> targets, List<List<String>> path, List<List<List<String>>> pathList){
        if(targets.size() == 0){
            pathList.add(new ArrayList<List<String>>(path));
            return;
        }

        if(!map.containsKey(id)){
            System.out.println("This is doesn't exist!");
            return;
        }

        List<String> list = new ArrayList<String>();
        list.add(id); // add current item id to the head of list
        list.addAll(map.get(id).idList);

        int size = list.size();
        for(int i=0; i<size; i++){

            if(i==0){
                Item cur = map.get(list.get(i));
                List<String> l = new ArrayList<String>();
                l.add(cur.id);
                if(targets.contains(cur.str)){
                    l.add(cur.str);
                    targets.remove(cur.str);
                }
                path.add(l);
            }
            // else if i is greater than 0, set this id in idList as visited by removing it from idList
            else if(map.containsKey(list.get(i))){
                System.out.println("i is greater than 0");
                Item curTemp = map.get(list.get(i));
                List<String> idListTemp = curTemp.idList;
                idListTemp.remove(list.get(i));
                map.remove(id);
                map.put(curTemp.id, new Item(curTemp.id, idListTemp, curTemp.str));

                List<String> l = new ArrayList<String>();
                l.add(map.get(curTemp.id).id);
                if(targets.contains(map.get(curTemp.id).str)){
                    l.add(map.get(curTemp.id).str);
                    targets.remove(map.get(curTemp.id).str);
                }
                path.add(l);
                if(i<size-1){
                    dfs(list.get(i+1), targets, path, pathList);
                    path.remove(path.size()-1);
                }

            }

        } // end for







    }

    public void buildInput(){
        List<String> temp1 = new ArrayList<String>();
        temp1.add("Donald");
        temp1.add("Barbara");
        Item it1 = new Item("Thomas", temp1, "a");
        map.put("Thomas", it1);

        List<String> temp2 = new ArrayList<String>();
        temp2.add("Sarah");
        temp2.add("Lisa");
        Item it2 = new Item("Donald", temp2, "");
        map.put("Donald", it2);


        List<String> temp3 = new ArrayList<String>();
        Item it3 = new Item("Lisa", temp3, "c");
        map.put("Lisa", it3);


        List<String> temp4 = new ArrayList<String>();
        temp4.add("Lisa");
        temp4.add("Thomas");
        temp4.add("Barbara");
        Item it4 = new Item("Sarah", temp4, "d");
        map.put("Sarah", it4);

        List<String> temp5 = new ArrayList<String>();
        temp5.add("Sarah");
        Item it5 = new Item("Barbara", temp5, "e");
        map.put("Barbara", it5);
    }

    public static void main(String[] args) {
        TestFinder tf = new TestFinder();
        tf.buildInput();

        List<String> targets = new ArrayList<String>();
        targets.add("a");
        targets.add("c");
        targets.add("d");
        tf.finder(targets, "Donald");

    }    
}

class Item{
    String id;
    List<String> idList;
    String str;


    Item(String i, List<String> il, String s){
        id = i;
        idList = il;
        str = s;
    }

}

However, now the result is empty, it couldn't list any sequence of items that can get target strings, which is obviously untrue for current situation.

I kinda got stuck here, I know there might something with my depth first search logic. Any inputs would be highly appreciated! Thank you.

Answer 1

(1) you else if block is incorrect (assuming that you don't have cyclical references)

before your first pass through for loop your list look like this: [Donald, Sarah, Lisa] once you iterator reaches i = 1 (Sarah) in the else if condition condition you check if Sarah has access to Sarah which will return false!

(2) this will not put current item on the head of the list:

    List<String> list = new ArrayList<String>();
    list.add(id); // add current item id to the head of list

This will:

    List<String> list = new ArrayList<String>();
    list.add(id); // add current item id to the head of list
    list.addAll(map.get(id).idList);

(3) if you want to iterate through all items it should be i < size (or i <= size-1)