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


##</code><code>##

            49         For alphabet and:        27 A
            /\         and weights as so:       15 B
          22  A                                  3 C
          /\                                     1 D
         7  B                                    1 E
        /\                                       1 F
       4  C                                      1 G
      /\
     2  2             Ordered so that the most frequent
    /\  /\            have the least 0's in their path
   D  EF  G

A=1   C=001   E=00001 G=00011
B=01  D=00000 F=00010

##</code><code>##

a 27 1
b 15 01
c 3  000
d 1  00100
e 1  00101
f 1  00110
g 1  00111

##</code><code>##

a 27 1
b 15 01
c 3  001
d 1  00000
e 1  00001
f 1  00010
g 1  00011