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The stable roommates problem is a well-known problem of matching n people into n/2 disjoint pairs so that no two unmatched persons both prefer each other to their partners under the matching. We call such a matching "a complete stable matching."It is known that a complete stable matching may not exist. Irving described an O(n') algorithm that would find one complete stable matching if there is one, or would report that none exists. In this paper, we give a necessary and sufficient condition for the existence of a complete stable matching; namely, the non-existence of any odd party, which will be defined subsequently. We define a new structure called a "stable partition," which generalizes the notion of a complete stable matching, and prove that every instance of the stable roommates problem has at least one such structure. We also show that a stable partition contains all the odd parties, if there are any. Finally we have an O(n*) algorithm that finds one stable partition which in turn gives all the odd parties. Q 1991 AcademicPress, Inc.

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