10
          /           \
8                        20
   \                         \
     4                          40
   /   \                     /
 3       6                 35
       /
     5

6  -> came from 5, coming from left, but 6 hasn't got right so go up
4  -> came from 6, coming from right, so go up
8  -> came from 4, coming from right, so go up
10 -> nor coming right, and 10 has got right, so make a right turn
20 -> not a leaf, hasn't got left go right
40 -> not a leaf, has got left go left
35 -> leaf so this is next leaf.