title: How to get the last row from DataFrame?
body: <p>I hava a DataFrame,the DataFrame hava two column 'value' and 'timestamp',,the 'timestmp' is ordered,I want to get the last row of the DataFrame,what should I do?</p>
  
  <p>this is my input:</p>
  
  <pre><code>+-----+---------+
  |value|timestamp|
  +-----+---------+
  |    1|        1|
  |    4|        2|
  |    3|        3|
  |    2|        4|
  |    5|        5|
  |    7|        6|
  |    3|        7|
  |    5|        8|
  |    4|        9|
  |   18|       10|
  +-----+---------+
  </code></pre>
  
  <p>this is my code:</p>
  
  <pre><code>    val arr = Array((1,1),(4,2),(3,3),(2,4),(5,5),(7,6),(3,7),(5,8),(4,9),(18,10))
      var df=m_sparkCtx.parallelize(arr).toDF("value","timestamp")
  </code></pre>
  
  <p>this is my expected result:</p>
  
  <pre><code>+-----+---------+
  |value|timestamp|
  +-----+---------+
  |   18|       10|
  +-----+---------+
  </code></pre>