Implementation

Once a problem is recognized as one of the four buckets from the intuition chapter, the code follows a fixed shape. These are the four skeletons, one per bucket, worth knowing from memory rather than re-derived each time.

Seen Before

function seenBefore(items) {
    const seen = new Set();
    for (const item of items) {
        if (seen.has(item)) return true; // already ran into this one
        seen.add(item);
    }
    return false;
}

Frequency

function frequency(items) {
    const counts = new Map();
    for (const item of items) {
        counts.set(item, (counts.get(item) || 0) + 1);
    }
    return counts;
}

Pairing

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

The check happens before the insert, not after. That order matters, it is what stops a value from pairing with itself.

Grouping

function grouping(items, keyFn) {
    const groups = new Map();
    for (const item of items) {
        const key = keyFn(item);
        if (!groups.has(key)) groups.set(key, []);
        groups.get(key).push(item);
    }
    return [...groups.values()];
}

keyFn is whatever computed property decides which bucket an item belongs to, a sorted string for anagrams, a row or column index for a grid, whatever the problem defines as “belongs together.”