title: Pandas swap values for columns
body: <p>What's the simplest way to achieve the below with pandas?</p>
  <pre><code>df1 = 
         A B C
   0     1 1 2
   1     2 3 1
   2     3 3 2
  </code></pre>
  <p>to</p>
  <pre><code>df_result = 
           1        2     3
   0     [A, B]    [C]    []
   1      [C]      [A]    [B]
   2      []       [C]   [A,B]  
  </code></pre>
  <p>Thanks in advance</p>