从单个链表中提取中间元素

Question

我有一个问题：

  

从单个链表中查找中间元素。

我需要知道这个问题的方法/方法。

Answer 1

您可以使用两个指针迭代列表 - 一个迭代速度是另一个的两倍。当快速指针到达列表的末尾时，慢速指针将指向中点。

算法：

public class MainActivity extends AppCompatActivity implements SwipeRefreshLayout.OnRefreshListener {

    private Toolbar toolbar;

    private TextView tvEmptyView;
    private RecyclerView mRecyclerView;
    private DataAdapter mAdapter;
    private LinearLayoutManager mLayoutManager;
    private RestManager mManager;
    private List<Feed> mFeed;
    SwipeRefreshLayout mSwipeRefreshLayout;
    protected Handler handler;
    private int currentPage=1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        tvEmptyView = (TextView) findViewById(R.id.empty_view);
        mRecyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);
        mSwipeRefreshLayout= (SwipeRefreshLayout) findViewById(R.id.swipe_refresh_layout);
        mSwipeRefreshLayout.setOnRefreshListener(this);
        //studentList = new ArrayList<Student>();
        mFeed = new ArrayList<Feed>();
        handler = new Handler();
        if (toolbar != null) {
            setSupportActionBar(toolbar);
            getSupportActionBar().setTitle("Android Students");

        }
        mManager = new RestManager();

        // use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView
        mRecyclerView.setHasFixedSize(true);

        mLayoutManager = new LinearLayoutManager(this);

        // use a linear layout manager
        mRecyclerView.setLayoutManager(mLayoutManager);

        // create an Object for Adapter
        mAdapter = new DataAdapter(mFeed,mRecyclerView);

        // set the adapter object to the Recyclerview
        mRecyclerView.setAdapter(mAdapter);
        //   mAdapter.notifyDataSetChanged();

        loadData(false);

        //        if (mFeed.isEmpty()) {
        //            mRecyclerView.setVisibility(View.GONE);
        //            tvEmptyView.setVisibility(View.VISIBLE);
        //
        //        } else {
        //            mRecyclerView.setVisibility(View.VISIBLE);
        //            tvEmptyView.setVisibility(View.GONE);
        //        }

        mAdapter.setOnLoadMoreListener(new OnLoadMoreListener() {
            @Override
            public void onLoadMore() {
                //add null , so the adapter will check view_type and show progress bar at bottom
                mFeed.add(null);
                mAdapter.notifyItemInserted(mFeed.size() - 1);

                handler.postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        //   remove progress item
                        mFeed.remove(mFeed.size() - 1);
                        // mAdapter.notifyItemRemoved(mFeed.size());
                        //add items one by one
                        int start = mFeed.size();
                        currentPage++;

                        Log.d("CurrentPage", String.valueOf(currentPage));
                        Call<Results> listCall = mManager.getFeedApi().getAllFeeds(1);

                        listCall.enqueue(new Callback<Results>() {

                            @Override
                            public void onResponse(Call<Results> call, Response<Results> response) {
                                mSwipeRefreshLayout.setRefreshing(false);
                                if (response.isSuccess()) {
                                    if (response.body() != null) {
                                        Results feedList = response.body();

                                        // List<Results> newUsers = response.body();

                                        Log.d("Retrofut", String.valueOf(feedList));

                                        for (int i = 0; i < feedList.results.size(); i++) {
                                            Feed feed = feedList.results.get(i);
                                            // mFeed.add(feed);
                                            mAdapter.addFeed(feed);
                                            //                                        mAdapter.notifyDataSetChanged();


                                            //mAdapter.notifyItemInserted(mFeed.size());

                                        }
                                        //    mAdapter.notifyDataSetChanged();
                                    }
                                }
                            }

                            @Override
                            public void onFailure(Call<Results> call, Throwable t) {
                                Log.d("Retrofut", "Error");
                                mFeed.remove(mFeed.size() - 1);
                                mAdapter.notifyItemRemoved(mFeed.size());

                                mAdapter.setLoaded();
                                mSwipeRefreshLayout.setRefreshing(false);
                            }
                        });

                        //        for (int i = 1; i <= 20; i++) {
                        //            studentList.add(new Student("Student " + i, "androidstudent" + i + "@gmail.com"));
                        //
                        //        }

                        mAdapter.setLoaded();
                        //or you can add all at once but do not forget to call mAdapter.notifyDataSetChanged();

                    }
                }, 2000);
            }
        });
    }

    // load initial data
    private void loadData(final boolean removePreData) {

        Call<Results> listCall = mManager.getFeedApi().getAllFeeds(1);

        listCall.enqueue(new Callback<Results>() {

                             @Override
                             public void onResponse(Call<Results> call, Response<Results> response) {

                                 if (response.isSuccess()) {
                                     if (response.body() != null) {
                                         //  if(removePreData) mAdapter.removeAll();
                                         Results feedList = response.body();
                                         Log.d("Retrofut", String.valueOf(feedList));

                                         for (int i = 0; i < feedList.results.size(); i++) {
                                             Feed feed = feedList.results.get(i);
                                             // mFeed.add(feed);
                                             //mAdapter.notifyDataSetChanged();
                                             mAdapter.addFeed(feed);
                                         }

                                         mSwipeRefreshLayout.setRefreshing(false);
                                     }
                                 }
                             }

                             @Override
                             public void onFailure(Call<Results> call, Throwable t) {
                                 Log.d("Retrofut", String.valueOf(t));
                                 mFeed.remove(mFeed.size() - 1);
                                 mAdapter.notifyItemRemoved(mFeed.size());
                                 mAdapter.setLoaded();
                                 mSwipeRefreshLayout.setRefreshing(false);
                             }
                         }
        );

        //        for (int i = 1; i <= 20; i++) {
        //            studentList.add(new Student("Student " + i, "androidstudent" + i + "@gmail.com"));
        //
        //        }

        mSwipeRefreshLayout.setRefreshing(true);
    }

    @Override
    public void onRefresh() {
        mFeed.clear();
        mAdapter.notifyDataSetChanged();
        loadData(true);
        currentPage=1;
    }
}