Hash Maps & Sets

Two Sum (unsorted)

For each number, check if its complement is already in the map. If not, store the number and its index. One pass, O(n).

Set operations

A Set is a Map without values — just keys. Three operations: add, has, delete. All O(1).

Templates

Map pattern

function twoSum( nums: number[], target: number) {
  const map = new Map<number, number>()

  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i]
    if (map.has(complement))
      return [map.get(complement)!, i]
    map.set(nums[i], i)
  }
}

Set pattern

function hasDuplicate( nums: number[]) {
  const seen = new Set<number>()

  for (const n of nums) {
    if (seen.has(n)) return true
    seen.add(n)
  }
  return false
}

Frequency counting

function topKFrequent( nums: number[], k: number) {
  const freq = new Map<number, number>()

  for (const n of nums)
    freq.set(n, (freq.get(n) ?? 0) + 1)

  return [...freq]
    .sort((a, b) => b[1] - a[1])
    .slice(0, k)
    .map(([num]) => num)
}