Atomicity is an important correctness condition for concurrent systems. Informally, atomicity is the property that every concurrent execution of a set of transactions is equivalent to some serial execution of the same transactions. In multi-threaded programs, executions of procedures (or methods) can be regarded as transactions. Correctness in the presence of concurrency often requires atomicity of these transactions. Tools that automatically detect atomicity violations can uncover subtle errors that are hard to find with traditional debugging and testing techniques. This paper presents new algorithms for runtime detection of violations of conflict-atomicity and view-atomicity, which are analogous to conflict-serializability and view-serializability in database systems. In these algorithms, the recorded events are formed into a graph with edges representing the synchronization within each transaction and possible interactions between transactions. We give conditions on the graph that imply conflict-atomicity and view-atomicity. Experiments show that these new algorithms are more efficient in most experiments and are more accurate than previous algorithms with comparable asymptotic complexity.