Triplets / k-sum (Two Pointers Sub-pattern)

// Triplets: sort, fix one element, then run opposite-ends two pointers on the rest.
vector<vector<int>> threeSum(vector<int> a, int target) {
    sort(a.begin(), a.end());
    vector<vector<int>> res;
    int n = a.size();
    for (int i = 0; i < n - 2; i++) {
        if (i > 0 && a[i] == a[i - 1]) continue;         // same fixed value → same triplets
        int left = i + 1, right = n - 1;
        while (left < right) {
            long long sum = (long long)a[i] + a[left] + a[right];
            if (sum < target) left++;
            else if (sum > target) right--;
            else {
                res.push_back({a[i], a[left], a[right]});
                while (left < right && a[left] == a[left + 1]) left++;    // skip duplicates
                while (left < right && a[right] == a[right - 1]) right--;
                left++, right--;
            }
        }
    }
    return res;
}