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public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        ListNode listNode;

        if (Objects.isNull(l1) && Objects.isNull(l2)) {
            return null;
        }

        if (l1.val > l2.val) {
            listNode = new ListNode(l2.val);
            l2 = l2.next;
        } else {
            listNode = new ListNode(l1.val);
            l1 = l1.next;
        }
        ListNode first = listNode;

        while (Objects.nonNull(l1.next) || Objects.nonNull(l2.next)) {
            if (l1.val < l2.val) {
                listNode.next = new ListNode(l1.val);
                l1 = l1.next;
            } else {
                listNode.next = new ListNode(l2.val);
                l2 = l2.next;
            }
            listNode = listNode.next;
        }

        if (l1.val > l2.val) {
            listNode.next = new ListNode(l2.val);
            listNode = listNode.next;
            listNode.next = new ListNode(l1.val);
        } else {
            listNode.next = new ListNode(l1.val);
            listNode = listNode.next;
            listNode.next = new ListNode(l2.val);
        }

        return first;
    }


2.

private List<Integer> path;
    private List<List<Integer>> paths;
    @Test
    void ttt() {
        int[][] arr = {{4,3,1}, {3,2,4}, {3}, {4}, {}};
        test(arr);
    }
    // 4,3,1  3,2,4  3  4  []
    public List<List<Integer>> test(int[][] graph) {
        path = new ArrayList<>();
        paths = new ArrayList<>();
        dfs(graph, 0);

        return paths;
    }

    public void dfs(int[][] graph, int current) {
        // 현재 포지션 저장
        path.add(current);
        if (current == graph.length - 1) {
            paths.add(new ArrayList<>(path));
        } else {
            for (final int i : graph[current]) {
                // 재귀
                dfs(graph, i);
            }
        }
        // 수행 후 포지션 뒤로
        path.remove(path.size() - 1);
    }


3. fail

class Solution {
    
    List<Integer> result;
    
    public boolean isSameTree(TreeNode p, TreeNode q) {
        result = new ArrayList<>();
        List<Integer> listP = search(p);

        result = new ArrayList<>();
        List<Integer> listQ = search(q);
        return listP.equals(listQ);
        
    }
    
    private List<Integer> search(TreeNode node) {
        result.add(node.val);
        while (Objects.nonNull(node.left)) {
            search(node.left);
        }

        while (Objects.nonNull(node.right)) {
            search(node.right);
        }

        return result;
    }
}
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