797. All Paths From Source to Target

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LeetCode

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Problem

Given a directed acyclic graph (DAG) of n nodes labeled from 0 to n - 1, find all possible paths from node 0 to node n - 1, and return them in any order.

The graph is given as follows: graph[i] is a list of all nodes you can visit from node i (i.e., there is a directed edge from node i to node graph[i][j]).

Example 1:

Input: graph = [[1,2],[3],[3],[]]
Output: [[0,1,3],[0,2,3]]
Explanation: There are two paths: 0 -> 1 -> 3 and 0 -> 2 -> 3.

Example 2:

Input: graph = [[4,3,1],[3,2,4],[3],[4],[]]
Output: [[0,4],[0,3,4],[0,1,3,4],[0,1,2,3,4],[0,1,4]]

Example 3:

Input: graph = [[1],[]]
Output: [[0,1]]

Example 4:

Input: graph = [[1,2,3],[2],[3],[]]
Output: [[0,1,2,3],[0,2,3],[0,3]]

Example 5:

Input: graph = [[1,3],[2],[3],[]]
Output: [[0,1,2,3],[0,3]]

Code

class Solution {
    List<List<Integer>> res;

    public List<List<Integer>> allPathsSourceTarget(int[][] graph) {
        res = new ArrayList<>();
        HashMap<Integer, List<Integer>> map = new HashMap<>();

        for (int i = 0; i < graph.length; i++) {
            map.put(i, new ArrayList<>());
            for (int nei : graph[i]) {
                map.get(i).add(nei);
            }
        }

        List<Integer> path = new ArrayList<>();
        path.add(0);
        helper(map, 0, graph.length - 1, path);
        return res;
    }

    private void helper(HashMap<Integer, List<Integer>> map, int curr, int target, List<Integer> path) {
        if (!map.containsKey(curr)) return;
        if (curr == target) {
            res.add(new ArrayList<>(path));
            return;
        }

        List<Integer> neis = map.get(curr);
        for (int nei : neis) {
            path.add(nei);
            helper(map, nei, target, path);
            path.remove(path.size() - 1);
        }
    }
}