Pairs

• Two Pointer Solution
• Time : O(nlogn)
• Space : O(1)

int pairs(int k, vector<int> A) {
    sort(begin(A), end(A));
    int res = 0, n = A.size(), l = 0, r = 0;
    
    while(r < n) {
        while(r < n and A[l]*1ll + k > A[r]) r++;
        if(A[l]*1ll + k == A[r]) {
            res++; l++, r++;
        } else l++;
    }
    
    return res;
}

• Hash Table Solution
• Time : O(n)
• Space : O(n)

int pairs(long k, vector<int> A) {
    unordered_set<long> us;
    int res = 0;
    for(auto& a : A) {
        if(us.count(a-k)) res++;
        if(us.count(a+k)) res++;
        us.insert(a);
    }

    return res;
}