Reply Actually it is only O n If you double the table at each step then for a given n the rehashing will have cost at most 1 2 n 4 n 2 n which is strictly less then 2 n thus the O n complexity The same holds for any exponential growth like increasing table size by 25 which would be more sensible for very large tables as you want to avoid large jumps in memory use for a very small variation of the input size The real proof can be done with geometric series