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How does a hash table work?

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Explain how a hash table (Python dict, JS Map) gives O(1) average lookups, and what happens on collisions.

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class TinyHashMap:
    def __init__(self, size=4):
        self.buckets = [[] for _ in range(size)]
    def put(self, key, value):
        bucket = self.buckets[hash(key) % len(self.buckets)]
        for pair in bucket:
            if pair[0] == key:
                pair[1] = value
                return
        bucket.append([key, value])
    def get(self, key):
        for k, v in self.buckets[hash(key) % len(self.buckets)]:
            if k == key:
                return v

m = TinyHashMap()
for i, word in enumerate(["ada", "bo", "cy", "dev", "eli"]):
    m.put(word, i)
print(m.get("dev"), [len(b) for b in m.buckets])

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