Create Approach.md

Author: cu-sanjay

Company_OA/Amazon_SDE_2025/Approach.md

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+### Approach
+
+We use two pointers: left and right to simulate buying books from both ends.
+
+- If pairCost is cheaper than buying both ends separately, and we still have k pair discounts left, we use it.
+- Otherwise, we buy the cheaper of the two ends individually.
+- If only one book remains, we buy it directly.
+
+### Edge Cases
+- If k = 0, we never use the pair option.
+- If pairCost >= cost[left] + cost[right], it's better to buy individually.
+
+### Tips
+- Always compare pairCost with cost[left] + cost[right].
+- Use long to avoid overflow for large cost arrays.
+
+### Time Complexity
+- **O(n)** – We process each book once.