Insert Delete GetRandom O(1)

Keep a dynamic array of values and a hash map from value to its current array index. Insert checks the map; if absent, append the value and record the new index. For delete, find the target index, take the last array value, write it into the target slot, update that moved value's map entry, then pop the array and delete the target map entry. This swap-with-last step avoids O(n) shifting while preserving a dense index range. getRandom chooses a uniformly random integer from 0 through length-1 and returns that array entry, so every stored value has equal probability. Explain the required order: handle deletion of the last value safely, and do not leave a stale moved index. Empty-set behavior should be explicit because random selection is undefined then. Expected insert, delete, and lookup are O(1) with a hash map; getRandom is O(1); space is O(n).