1. A method for designing an alternating phase shift mask (altPSM), the method comprising: providing an initial feature layout of design elements to be printed, wherein said initial feature layout does not include phase shift shapes; providing a layout rules set for designing a conflict-free altPSM layout; identifying critical elements of said design elements, wherein each of said critical elements has a critical dimension; assigning a feature node corresponding to each of said critical elements; assigning phase shape representors disposed on opposing sides of each of said critical dimensions; assigning a connection node between interacting adjacent phase representors which interact in accordance with an adjacency rules set; constructing a planar interlock graph comprising said feature nodes, said connection nodes and connection paths between said feature nodes and said connection nodes; analyzing said interlock graph to identify a violation, if any, of said layout rules set; if a violation of said layout rules set exists, generating at least one layout constraint to correct said violation and form a violation-free feature layout; and assigning phase shift shapes corresponding to said phase representors in said violation-free feature layout in accordance with said layout rules set and with said at least one layout constraint to form a conflict-free altPSM layout.