Resolve a Hash Bucket

Practice Exercise Interview Easy Hashing

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Problem

Implement resolve_hash_bucket(buckets, key). buckets is a list of collision chains; each chain contains [stored_key, value] pairs. Select key % len(buckets), scan that bucket, and return the matching value or None.

Starter code

def resolve_hash_bucket(buckets, key):
    pass

Test cases

collision-chain

{
  "args": [
    [
      [],
      [
        [
          1,
          "one"
        ],
        [
          5,
          "five"
        ]
      ],
      [],
      []
    ],
    5
  ]
}

Expected: "five"

missing-collision

{
  "args": [
    [
      [],
      [
        [
          1,
          "one"
        ]
      ],
      [],
      []
    ],
    9
  ]
}

Expected:

Wizard outline

Step 1: Select one deterministic bucket
Use key modulo bucket count and read the only entry in a single-entry bucket. Bucket selection is the constant-time routing decision that makes a full-table scan unnecessary.
Step 2: Compare full keys in a collision chain
Scan only the selected bucket and return the value whose stored key exactly equals key. Modulo identifies a bucket, not a unique key; collisions require exact equality inside that bucket.
Step 3: Represent an absent key
Return None after the complete selected collision chain has no exact match. The final contract must distinguish a routed-but-missing key from an execution failure.

Footguns and prerequisites

Returning the first bucket entry ignores collisions between different keys.
Using key directly as a bucket index fails as soon as the key exceeds the bucket count.
hashing and sets

Reviewed references

Python Mapping Types — dict

Prepared Interview Problems

Design HashMap(opens in a new tab)
Resolve a Hash Bucket isolates only entries in the computed bucket can match, and an entry matches only when stored_key equals key. That focused state discipline is required when implementing design hashmap as a complete Interview Problem.
Design HashSet(opens in a new tab)
Resolve a Hash Bucket isolates only entries in the computed bucket can match, and an entry matches only when stored_key equals key. That focused state discipline is required when implementing design hashset as a complete Interview Problem.

Recommended approach and implementation

A hash narrows lookup to one bucket, but collisions still require comparing the complete stored key. Only entries in the computed bucket can match, and an entry matches only when stored_key equals key.

Why it works: Equal keys always compute the same bucket index, so searching that bucket cannot miss a match. Comparing stored keys prevents collisions from producing false matches, yielding the associated value or the missing sentinel.

def resolve_hash_bucket(buckets, key):
    bucket = buckets[key % len(buckets)]
    for stored_key, value in bucket:
        if stored_key == key:
            return value
    return None