Add Two Sum solution in Java (kumar1593#12)

[maitriupadhyay03-cell]

Closes #12

Summary

This PR adds a Two Sum solution in Java to the Arrays section.

Problem

Given an array of integers and a target, return indices of the two numbers that add up to the target.

Approach

HashMap-based single-pass solution:

• For each element, check if (target - element) is already in the map
• • If found, return both indices
• • Otherwise, store the element and its index

Complexity

• Time: O(n)
• • Space: O(n)

Test Cases

• [2,7,11,15], target=9 → [0,1]
• • [3,2,4], target=6 → [1,2]
• • [3,3], target=6 → [0,1]

[maitriupadhyay03-cell]

Fix applied: Renamed two_sum.java → TwoSum.java and updated the public class name from two_sum to TwoSum to follow standard Java PascalCase naming conventions. Also updated the object instantiation in main() accordingly. The logic remains unchanged.