expected number of elements
ratio of n to the number of internal buckets. The desired (approximate) number of elements per bucket. For example, 0.5 sets the number of buckets to double n; for 2 the number of buckets is the half of n. load_factor must be greater than 0. The load factor is basically a trade-off between memory usage (number of buckets) and search time (number of elements per bucket).
Constructor, sets the number of buckets to n * load_factor