根节点到叶子节点的节点值之和等于\ sum sum

public ArrayList<ArrayList<Integer>> pathSum (TreeNode root, int sum) {
        // write code here
        ArrayList<Integer> list = new ArrayList<>();
        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();
        dfs(lists,list,0,sum,root);
        return lists;
    }
    private void dfs(ArrayList<ArrayList<Integer>> lists, ArrayList<Integer> list,
                    int sum ,int k,TreeNode root){
        if(root==null) return;
        sum  += root.val;
        list.add(root.val);
        if(root.left==null && root.right==null){
            if(sum==k){
                ArrayList<Integer> gg  = new ArrayList<>();
                for(int l:list){
                    gg.add(l);
                }
                //*注意list里面嵌入list的时候 改变里面的list会改变外层list 即添加进去的是引用*
                lists.add(gg);
            }
            list.remove(list.size()-1);
            return;
        }
        dfs(lists,list,sum,k,root.left);
        dfs(lists,list,sum,k,root.right);
        list.remove(list.size()-1);
    }