search(target), start from the top level and move right until the next node's value is greater than target, then go down one level and continue until the bottom level is reached. Return true if target is found, false otherwise.add(num), perform a similar traversal as search but keep track of the predecessors at each level. Insert num after the predecessors, randomly deciding whether to insert it at higher levels.erase(num), again traverse the list to find num, and remove it by adjusting pointers at each level where it appears.