Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 13 | C++
13.2 Compare and contrast a hash table vs. an STL map. How is a hash table implemented?
If the number of inputs is small, what data structure options can be used instead of
a hash table?
SOLUTION
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Compare and contrast Hash Table vs. STL map
In a hash table, a value is stored by applying hash function on a key. Thus, values are not
stored in a hash table in sorted order. Additionally, since hash tables use the key to find the
index that will store the value, an insert/lookup can be done in amortised O(1) time (assuming
only a few collisions in the hashtable). One must also handle potential collisions in a
hashtable.
In an STL map, insertion of key/value pair is in sorted order of key. It uses a tree to store
values, which is why an O(log N) insert/lookup is required. There is also no need to handle
collisions. An STL map works well for things like:
»»
find min element
»»
find max element
»»
print elements in sorted order
»»
find the exact element or, if the element is not found, find the next smallest number
How is a hash table implemented?
1. A good hash function is required (e.g.: operation % prime number) to ensure that the
hash values are uniformly distributed.
2. A collision resolving method is also needed: chaining (good for dense table entries),
probing (good for sparse table entries), etc.
3. Implement methods to dynamically increase or decrease the hash table size on a given
criterion. For example, when the [number of elements] by [table size] ratio is greater
than the fixed threshold, increase the hash table size by creating a new hash table and
transfer the entries from the old table to the new table by computing the index using
new hash function.
What can be used instead of a hash table, if the number of inputs is small?
You can use an STL map. Although this takes O(log n) time, since the number of inputs is
small, this time is negligible.
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