Designing a hashset with LinkedLists as buckets

Question

I am solving a problem on Leetcode that requires me to design a hash set. I intend to solve it using linked lists where each linked list would represent a bucket.

I am intending to implement this: enter image description here

However, I am failing in one large test case.

This is my solution that failed a test case mentioned below:

class MyHashSet:
 def __init__(self):
 """
 Initialize your data structure here.
 """
 self.key_range = 769
 self.buckets = [Bucket() for _ in range(self.key_range)]
 def add(self, key: int) -> None:
 _hash_ = self._hash(key)
 bucket = self.buckets[_hash_]
 bucket.insert(key)
 def _hash(self, key):
 return key % self.key_range
 def remove(self, key: int) -> None:
 _hash_ = self._hash(key)
 bucket = self.buckets[_hash_]
 bucket.delete(key)
 def contains(self, key: int) -> bool:
 """
 Returns true if this set contains the specified element
 """
 _hash_ = self._hash(key)
 bucket = self.buckets[_hash_]
 return bucket.check_val(key)
class Node:
 def __init__(self, val):
 self.val = val
 self.next = None
class Bucket:
 def __init__(self):
 self.head = Node('*')
 self.curr = self.head
 def insert(self, val):
 if not self.check_val(val):
 self.curr.next = Node(val)
 self.curr = self.curr.next
 def delete(self, val):
 curr = self.head.next
 prev = self.head
 # found = False
 while curr is not None:
 if curr.val == val:
 prev.next = curr.next
 return
 prev = curr
 curr = curr.next
 def check_val(self, val):
 curr = self.head.next
 while curr:
 if curr.val == val:
 return True
 curr = curr.next
 return False

The failed test case is:

Input:

["MyHashSet","contains","remove","add","add","add","add","contains","add","add","contains","contains","add","contains","add","add","add","contains","remove","add","add","contains","add","add","add","add","remove","add","add","add","add","contains","add","add","contains","contains","contains","remove","add","add","contains","remove","remove","add","add","add","add","add","contains","remove","contains","add","contains","add","contains","remove","add","add","remove","contains","add","add","add","add","add","add","add","contains","remove","add","add","remove","remove","add","remove","add","remove","add","add","add","add","add","add","add","remove","add","remove","remove","add","add","add","add","contains","add","add","remove","add","add","add","remove","add","add","add","add","contains","add","add","add","remove","contains","add","add","add","add","add","remove","add","contains","contains","add","add","add","add","contains","remove","add","contains","add","contains","contains","add","add","contains","add","add","add","add","add","add","add","contains","remove","add","contains","add","remove","add","remove","remove","add","add","contains","add","remove","contains","add","add","add","add","add","add","add","contains","remove","remove","contains","add","contains","remove","add","add","add","add","contains","add","remove","add","remove","add","remove","add","add","add","remove","contains","add","add","add","add","remove","add","add","contains","contains","contains","remove","contains","remove","add","contains","add","add","add","add","remove","add","add","add","remove","remove","add","add","add","add","remove","add","remove","contains","add","contains","add","contains","add","contains","add","add","add","remove","add","add","add","add","add","contains","add","add","add","add","add","add","remove","add","remove","add","add","add","contains","add","add","remove","remove","remove","contains","remove","add","add","remove","add","contains","add","add","remove","contains","add","add","add","contains","remove","add","add","remove","contains","remove","add","contains","remove","add","add","add","add","contains","add","contains","add","remove","contains","add","add","add","add","add","add","add","remove","contains","contains","add","remove","add","remove","contains","add","contains","add","remove","add","add","add","add","remove","add","add","add","add","add","remove","contains","add","add","remove","contains","add","add","contains","add","remove","add","remove","add","add","remove","contains","add","add","add","contains","add","add","add","remove","add","remove","contains","add","add","contains","add","contains","add","add","remove","add","contains","contains","remove","contains","add","contains","add","add","remove","remove","add","add","remove","add","contains","add","add","add","contains","add","add","add","add","add","contains","add","add","contains","add","add","add","remove","add","add","add","add","add","add","add","add","remove","add","add","contains","remove","add","remove","contains","add","add","add","remove","add","add","contains","contains","add","contains","add","add","remove","add","add","remove","remove","add","add","add","contains","add","add","remove","add","remove","add","add","add","contains","add","add","add","add","contains","add","remove","add","add","remove","add","add","contains","add","add","add","add","contains","add","contains","add","add","add","add","add","remove","remove","remove","remove","contains","add","add","add","contains","add","contains","remove","add","add","contains","add","add","contains","add","contains","add","remove","add","remove","add","add","contains","add","add","add","add","add","add","add","add","add","add","add","add","add","contains","add","add","remove","add","add","remove","add","remove","remove","remove","contains","remove","contains","remove","add","remove","add","add","remove","add","add","remove","add","add","remove","add","contains","remove","add","contains","contains","remove","add","contains","add","add","contains","contains","add","add","add","add","contains","add","contains","add","add","remove","add","remove","contains","add","remove","remove","add","add","contains","contains","contains","add","contains","add","add","remove","add","add","add","remove","add","add","add","add","contains","remove","remove","contains","add","add","add","add","add","add","remove","contains","remove","add","add","add","contains","add","remove","add","add","remove","add","add","add","add","remove","remove","remove","add","contains","add","add","add","add","add","add","add","remove","add","add","add","add","add","add","add","remove","add","add","add","add","remove","add","add","add","contains","add","remove","add","add","add","add","add","add","add","add","add","contains","remove","remove","contains","add","remove","add","contains","add","remove","add","contains","add","add","add","add","contains","add","add","contains","contains","contains","add","remove","contains","add","add","rem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Output:

[null,false,null,null,null,null,null,false,null,null,false,true,null,false,null,null,null,true,null,null,null,false,null,null,null,null,null,null,null,null,null,true,null,null,false,true,true,null,null,null,true,null,null,null,null,null,null,null,true,null,false,null,true,null,false,null,null,null,null,false,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,null,true,null,null,null,null,null,null,null,true,false,null,null,null,null,false,null,null,false,null,false,false,null,null,true,null,null,null,null,null,null,null,false,null,null,false,null,null,null,null,null,null,null,false,null,null,true,null,null,null,null,null,null,null,false,null,null,false,null,true,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,true,false,false,null,true,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,false,null,false,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,false,null,null,null,null,null,false,null,null,null,false,null,null,null,true,null,null,null,null,true,null,null,false,null,null,null,null,null,true,null,true,null,null,true,null,null,null,null,null,null,null,null,true,false,null,null,null,null,false,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,false,null,null,true,null,null,null,null,null,null,null,true,null,null,null,false,null,null,null,null,null,null,true,null,null,true,null,true,null,null,null,null,false,true,null,true,null,true,null,null,null,null,null,null,null,null,false,null,null,null,false,null,null,null,null,null,true,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,null,null,null,null,null,true,false,null,true,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,false,null,null,null,null,true,null,null,null,null,null,null,null,true,null,null,null,null,true,null,false,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,false,null,null,null,true,null,null,true,null,false,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,null,null,null,null,null,null,null,true,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,true,true,null,null,true,null,null,false,false,null,null,null,null,true,null,false,null,null,null,null,null,true,null,null,null,null,null,false,true,false,null,true,null,null,null,null,null,null,null,null,null,null,null,true,null,null,true,null,null,null,null,null,null,null,true,null,null,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,true,null,null,true,null,null,null,true,null,null,null,true,null,null,null,null,true,null,null,false,false,false,null,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,true,false,null,null,null,null,null,null,null,null,false,null,null,null,null,null,true,null,true,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,false,null,true,false,null,null,null,null,false,null,null,null,null,null,null,false,null,null,null,false,null,null,null,null,null,null,null,null,null,null,true,null,null,null,true,null,false,true,null,null,null,null,null,null,null,true,null,null,true,null,null,false,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,true,null,null,null,true,null,null,null,false,null,null,null,null,false,true,null,false,null,null,null,false,null,null,null,true,null,null,false,true,null,null,true,false,null,null,null,true,null,null,null,true,null,null,null,null,true,null,null,null,null,null,null,null,true,null,null,false,true,null,null,null,true,null,null,false,null,null,true,false,null,null,null,null,null,null,null,false,null,true,null,null,null,null,null,false,null,null,null,null,null,null,null,false,false,null,null,null,null,null,null,null,null,false,true,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,null,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,true,null,null,null,null,null,false,null,null,null,null,null,null,false,null,false,null,null,true,null,null,null,null,null,null,null,null,null,null,null,true,false,null,false,null,true,null,null,null,null,null,null,null,null,true,null,null]

Expected Output:

[null,false,null,null,null,null,null,false,null,null,false,true,null,false,null,null,null,true,null,null,null,false,null,null,null,null,null,null,null,null,null,true,null,null,false,true,true,null,null,null,true,null,null,null,null,null,null,null,true,null,false,null,true,null,false,null,null,null,null,false,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,null,true,null,null,null,null,null,null,null,true,false,null,null,null,null,false,null,null,false,null,false,false,null,null,true,null,null,null,null,null,null,null,false,null,null,false,null,null,null,null,null,null,null,false,null,null,true,null,null,null,null,null,null,null,false,null,null,false,null,true,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,true,false,false,null,true,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,false,null,false,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,false,null,null,null,null,null,false,null,null,null,false,null,null,null,true,null,null,null,null,true,null,null,false,null,null,null,null,null,true,null,true,null,null,true,null,null,null,null,null,null,null,null,true,false,null,null,null,null,false,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,false,null,null,true,null,null,null,null,null,null,null,true,null,null,null,false,null,null,null,null,null,null,true,null,null,true,null,true,null,null,null,null,false,true,null,true,null,true,null,null,null,null,null,null,null,null,false,null,null,null,false,null,null,null,null,null,true,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,null,null,null,null,null,true,false,null,true,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,false,null,null,null,null,true,null,null,null,null,null,null,null,true,null,null,null,null,true,null,false,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,false,null,null,null,true,null,null,true,null,false,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,null,null,null,null,null,null,null,true,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,true,true,null,null,true,null,null,false,false,null,null,null,null,true,null,false,null,null,null,null,null,true,null,null,null,null,null,false,true,false,null,true,null,null,null,null,null,null,null,null,null,null,null,true,null,null,true,null,null,null,null,null,null,null,true,null,null,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,true,null,null,true,null,null,null,true,null,null,null,true,null,null,null,null,true,null,null,false,true,false,null,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,true,false,null,null,null,null,null,null,null,null,false,null,null,null,null,null,true,null,true,null,null,null,null,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,true,null,null,null,null,false,null,true,false,null,null,null,null,false,null,null,null,null,null,null,false,null,null,null,false,null,null,null,null,null,null,null,null,null,null,true,null,null,null,true,null,false,true,null,null,null,null,null,null,null,true,null,null,true,null,null,false,null,null,null,true,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,true,null,null,null,true,null,null,null,false,null,null,null,null,false,true,null,false,null,null,null,false,null,null,null,true,null,null,false,true,null,null,true,false,null,null,null,true,null,null,null,true,null,null,null,null,true,null,null,null,null,null,null,null,true,null,null,false,true,null,null,null,true,null,null,false,null,null,true,false,null,null,null,null,null,null,null,false,null,true,null,null,null,null,null,false,null,null,null,null,null,null,null,false,false,null,null,null,null,null,null,null,null,false,true,null,null,false,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,false,null,null,null,true,null,null,null,true,null,null,null,null,null,null,null,null,null,true,null,null,null,null,null,true,null,null,null,null,null,false,null,null,null,null,null,null,false,null,false,null,null,true,null,null,null,null,null,null,null,null,null,null,null,true,false,null,false,null,true,null,null,null,null,null,null,null,null,true,null,null]

I then took a look at the solution and figured out that it intends to add a new node at the head, whereas I append the new node to the tail. Here, the node represents a unique value in the bucket. In my opinion, both the ways should work and so says the author:

For a value that was never seen before, we insert it to the head of the bucket, though we could also append it to the tail. It is a choice that we made, which could fit better the scenario where redundant values are operated in nearby time windows, since it is more likely that we spot the value at the head of the bucket rather than walking through the entire bucket.

The solution that worked:

class MyHashSet:
 def __init__(self):
 """
 Initialize your data structure here.
 """
 self.key_range = 769
 self.buckets = [Bucket() for _ in range(self.key_range)]
 
 def _hash(self, key):
 return key % self.key_range
 
 def add(self, key: int) -> None:
 bucket_index = self._hash(key)
 self.buckets[bucket_index].insert(key)
 def remove(self, key: int) -> None:
 bucket_index = self._hash(key)
 self.buckets[bucket_index].delete(key)
 def contains(self, key: int) -> bool:
 """
 Returns true if this set contains the specified element
 """
 bucket_index = self._hash(key)
 return self.buckets[bucket_index].check_val(key)
class Node:
 def __init__(self, val, next_node=None):
 self.val = val
 self.next = next_node
class Bucket:
 def __init__(self):
 self.head = Node('*')
 
 def insert(self, val):
 if not self.check_val(val):
 self.head.next = Node(val, self.head.next)
 def delete(self, val):
 curr = self.head.next
 prev = self.head
 # found = False
 while curr is not None:
 if curr.val == val:
 prev.next = curr.next
 return
 prev = curr
 curr = curr.next
 def check_val(self, val):
 curr = self.head.next
 while curr:
 if curr.val == val:
 return True
 curr = curr.next
 return False

I failed to see what is wrong with appending values at the end.

• superb rain: codereview.stackexchange.com/help/dont-ask

Answer 1

I found the bug. It was in the delete function.

I was wrongly marking the self.curr pointer.

The correction is:

def delete(self, val):
 curr = self.head
 prev = self.head
 while curr is not None:
 if curr.val == val:
 prev.next = curr.next
 break
 prev = curr
 curr = curr.next
 if not curr or not curr.next: # we reached the last node
 # so that curr points to last valid node, whether the prev last was deleted, or this would be a redundant.
 self.curr = prev

My self.curr still points to the deleted node.

The cases that I missed: correctly marking the self.curr in the cases of last node deletion.