Common Mistakes

• Wrong tree size: Segment tree needs up to $4$n nodes for safety. 2n might not suffice for all indices.
• Incorrect range handling: Off-by-one errors in range queries are common. Carefully define inclusive/exclusive bounds.
• Forgetting to propagate lazy updates: With lazy propagation, must push updates down before querying children.
• Integer overflow in sums: Large arrays with large values can overflow.

Use long integers.