CareerCup The number of pairs (x, y) of distinct elements with condition x + y <= Threshold

Problem1: You have a lists with integers. Find all the pairs of numbers that sum less than or equal to to a particular number k. The
list contains minimum 5 Million number.

(I provided a n^2logn solution but they may be looking forward to having a better answer).

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Problem2:
Input - array of integers size N, integer Threshold. Output - the number of pairs (x, y) of distinct elements with condition x + y <= Threshold. Is that possible to implement it with O(n) ?

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// perform radix sort - O(n). now the array is sorted Or quicksort, O(n) is difficult
// now, remove duplicates if you wish. and now perform the following algorithm
int i(0), j(n-1), ans(0);
while (i < j)
{
       if (a[i] + a[j] <= threshold ) {
                ans += (j-i);
                i++; 
       }
       else { j--; } 
}
return ans;