The Huffman algorithm pairs the sub-trees by weight. As long as there are more than one sub-tree, pairs the lightest two into one sub-tree. I chose to have the lightest branches on the left (bit 0). 

            49           For a file containing: 27 A
            /\                                  15 B
          22  A                                  3 C
          /\                                     1 D
         7  B                                    1 E
        /\                                       1 F
       C  4                                      1 G
         /\
        2  2             the resulting tree is (with the weight of each
       /\  /\            sub-tree noted at its root) drawn on the left.
      D  EF  G