Figure 3

An example of a PQ-tree. An example of a PQ tree. Circles represent P nodes and rectangles represent Q nodes. Figure 3(a): Initial PQ tree T formed from set {a, b, c, d}. Figure 3(b): The PQ tree T after the REDUCE(T,{a, c}) operation, requiring that a and c be consecutive. Figure 3(c): The PQ tree T after the REDUCE(T,{b,c}) operation, requiring that b and c be consecutive. Valid permutations represented by this tree are the sequences acbd, bcad, dacb, and dbca.