Introduction:
When preparing for a competitive examination like UPSC, solving previous years' papers is an essential part of the preparation process. It helps in understanding the exam pattern, identifying important topics, and practicing time management. However, whether to solve the papers subject-wise or topic-wise depends on various factors. Let's analyze both approaches in detail.

Subject-wise Approach:
Solving previous years' papers subject-wise means focusing on one subject at a time and solving all the questions related to that subject from different years' papers. This approach has its advantages:

1. Thorough Understanding: Solving subject-wise papers allows candidates to gain a comprehensive understanding of each subject individually. It helps in identifying the areas of strength and weakness in a specific subject.

2. Exam Pattern Familiarity: By solving subject-wise papers, candidates become familiar with the exam pattern of each subject. They can analyze the types of questions asked, the weightage given to different topics, and the overall difficulty level.

3. Topic-wise Revision: While solving subject-wise papers, candidates can revise the related topics simultaneously. It helps in reinforcing the concepts and retaining the information for a longer duration.

4. Time Management: Solving subject-wise papers allows candidates to practice time management effectively. They can allocate a specific time duration for each subject and work on improving their speed and accuracy.

Topic-wise Approach:
Solving previous years' papers topic-wise means selecting a specific topic from any subject and solving all the questions related to that topic from different years' papers. This approach also has its advantages:

1. Targeted Preparation: Solving topic-wise papers helps candidates focus on specific areas of each subject. They can identify the important topics and allocate more time for their preparation accordingly.

2. Concept Clarity: By solving topic-wise papers, candidates can enhance their conceptual clarity on individual topics. It allows them to delve deeper into each topic, understand the underlying principles, and solve related questions more effectively.

3. Identifying Patterns: Solving topic-wise papers enables candidates to identify patterns and trends in the questions asked from a particular topic. It helps in understanding the examiner's mindset and preparing accordingly.

4. Efficient Revision: Solving topic-wise papers facilitates efficient revision as candidates can revise specific topics without getting overwhelmed by the entire subject. It saves time and helps in retaining the information better.

Conclusion:
Both subject-wise and topic-wise approaches have their own merits. However, it is advisable to adopt a combination of both approaches for comprehensive preparation. Candidates can start with subject-wise papers to build a strong foundation and gain a thorough understanding of each subject. Later, they can switch to topic-wise papers to focus on specific areas, reinforce concepts, and improve their problem-solving skills. Regular practice, analysis of previous years' papers, and effective time management are key to success in UPSC examinations.