Add two pointer solution

[P-1256]

• File(s) Modified: cpp/0001-two-sum.cpp
• Language(s) Used: C++
• Submission URL: https://leetcode.com/problems/two-sum/submissions/1867274302/

Added an alternative approach using sorting and two pointers.

• Time Complexity: O(n log n)
• Space Complexity: O(n)

Ensures indices are returned in sorted order.