## Hashing

Trade O(n) memory for **O(1) average lookup**. The workhorse behind "have I seen this before?".

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## Recognition

- "Does X exist?" / "How many times?" → `Set` / `Map`
- Pair matching (complement lookup) → map value → index
- Grouping by a derived key → map key → bucket
- Counting characters → frequency map

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## Template — complement lookup (LC 1)

```js
function twoSum(nums, target) {
  const seen = new Map(); // value -> index
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
}
```

One pass: check for the partner **before** inserting yourself.

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## Template — grouping (LC 49)

```js
function groupAnagrams(strs) {
  const groups = new Map();
  for (const s of strs) {
    const key = [...s].sort().join("");
    if (!groups.has(key)) groups.set(key, []);
    groups.get(key).push(s);
  }
  return [...groups.values()];
}
```

The whole trick is **choosing the canonical key**.

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## Pitfalls

- Frequency compare needs both directions (surplus *and* deficit)
- Object keys coerce to strings — use `Map` for numbers
- Hashing is O(1) *average*; keys must be cheap to compute

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## Recall

- Two Sum: why insert after the lookup, not before?
- Group Anagrams: name two valid canonical keys
- LC 128 Longest Consecutive: why check `!set.has(x - 1)` first?

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## Drill

- 217 Contains Duplicate · 242 Valid Anagram
- 1 Two Sum · 49 Group Anagrams
- 347 Top K Frequent · 383 Ransom Note
- 205 Isomorphic Strings · 128 Longest Consecutive · 290 Word Pattern