Insert Delete GetRandom O(1)

Use two structures that keep each other consistent: an array containing every set member exactly once, and a map from member to its array position. Inserting a new value appends it and records its index; inserting an existing value returns false. To remove a value, locate its index in the map, copy the current last array value into that index, update the copied value's index, pop the final slot, and remove the deleted value's map entry. When the deleted value already is last, the same logic works if updates are ordered carefully. The invariant is that every map entry points to exactly one live array element and every array element maps back to its index. Random selection is uniform because the array is dense and a uniform random index gives each member probability 1/n. Empty random requests need a documented failure behavior. Hash-based operations are O(1) expected time and O(n) space.