find pair of numbers in array that add to given sum

Question

Question: Given an unsorted array of positive integers, is it possible to find a pair of integers from that array that sum up to a given sum?

Constraints: This shou

Answer 1

1. Use count sort to sort the array O(n).

2. take two pointers one starts from 0th index of array, and another from end of array say (n-1).

   run the loop until low <= high

   Sum = arr[low] + arr[high]  
   if(sum == target)
          print low, high
   if(sum < target)
          low++
   if(sum > target)
          high--
   

   Step 2 to 10 takes O(n) time, and counting sort takes O(n). So total time complexity will be O(n).