Common DSA Mistakes and How to Avoid Them

Off-by-One and Index Errors

• Binary search: mid = (lo + hi) / 2 can overflow; use lo + (hi - lo) / 2. Check lo <= hi vs lo < hi and when to return.
• Array bounds: 0-indexed vs 1-indexed; check loop start and end.

Complexity Analysis

• Claiming O(1) when you have a loop. Count nested loops and recursive calls.
• Saying "hash map lookup is O(1)" without noting worst-case O(n) for collisions. Usually assume average O(1).

Not Clarifying the Problem

• Input range: size of array, value range. Can you modify the array?
• Output: single answer or all? Sorted or any order?
• Edge cases: empty input, one element, duplicates.

Coding Too Fast

• Think of a simple example and walk through your algorithm first.
• Write pseudocode or main steps before coding.
• Test with a small example before saying "done."

Summary

Slow down, clarify, check indices and bounds, and state complexity clearly. Use Preplume(/) to practice under timed conditions and build good habits.