Unique topological ordering

Given n items labeled 0..n-1 and a list of ordering constraints [a, b] meaning a must come before b, determine whether the constraints pin down exactly ONE valid total ordering of all items. Return True if a unique topological order exists, and False if there are zero valid orders (a cycle) or more than one. Duplicate constraints should be treated as a single constraint. There are at most 10^4 items.