leetcode/islands/decks/hashing.md

Hashing

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

---

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

---

Template — complement lookup (LC 1)

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.

---

Template — grouping (LC 49)

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.

---

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

---

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?

---

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