Editing PQ tree (section)

==Examples and notation==
[[Image:pq-tree-5-leaves.svg|left|thumb|The PQ tree representing 
<br/>[1 (2 3 4) 5] ]]

If all the leaves of a PQ tree are connected directly to a root P node then all possible orderings are allowed. If all the leaves are connected directly to a root Q node then only one order and its reverse are allowed. If nodes a,b,c connect to a P node, which connects to a root P node, with all other leaf nodes connected directly to the root, then any ordering where a,b,c are contiguous is allowed.

Where graphical presentation is unavailable PQ trees are often noted using nested parenthesized lists.  Each matched pair of square parentheses represents a Q node and each matched pair of rounded parentheses represent a P node.  Leaves are non-parentheses elements of the lists.  The image on the left is represented in this notation by [1 (2 3 4) 5]. This PQ tree represents the following twelve permutations on the set {1, 2, 3, 4, 5}:
: 12345, 12435, 13245, 13425, 14235, 14325, 52341, 52431, 53241, 53421, 54231, 54321.