Introduction

First Bad Version means:

  • finding earliest bad version
    inside ordered versions list

Rule:

If a version is bad, all versions after it are also bad.

Goal:

  • find first bad version
    using minimum API calls

Example:

Versions:
1 2 3 4 5

Bad starts at:
4

Output:
4

Explanation:

Version 4 is the first version that becomes bad.

This problem is one of the most important applications of:

Binary Search

Constraints

1 <= n <= 2^31 - 1

Approach : Binary Search Optimization

Explanations:

Explanation:

The idea is:

  • versions are sorted
    by good and bad pattern
  • use binary search
    to minimize checks

Pattern:

Good Good Good Bad Bad Bad

Steps:

  1. Initialize left pointer.
  2. Initialize right pointer.
  3. Find middle version.
  4. Check if middle is bad.
  5. Move search range.
  6. Continue until first bad found.

Conditions:

isBadVersion(mid) == true → move left 

isBadVersion(mid) == false → move right

This approach:

  • reduces API calls
  • uses divide and conquer

Dry Run

Versions:1 2 3 4 5

Middle:
3
Version 3 is good.
Move right.

Middle:
4
Version 4 is bad.
Move left.

Answer:
4

Practice :

Complexity Analysis :

Time Complexity:- O(log n)
Explanation :
Search space reduces by half every iteration.

Space Complexity:- O(1)
Explanation :
Only constant variables are used.

Why This Problem is Important

This problem builds the foundation for:

  • Binary search optimization
  • API minimization
  • Divide and conquer
  • Efficient searching
  • Boundary searching problems

Real-World Applications

First bad version concepts are used in:

  • Software testing
  • Deployment systems
  • Version control
  • Debugging systems
  • Quality assurance pipelines

Common Beginner Mistakes

  • Infinite loop conditions
  • Incorrect middle updates
  • Wrong boundary movement
  • Missing first occurrence logic
  • Overflow in middle calculation

Interview Tip

Interviewers often expect:

  • binary search understanding
  • boundary searching explanation
  • API optimization discussion
  • overflow-safe middle calculation

Always explain:

  • monotonic bad pattern
  • search space reduction
  • first occurrence detection

Related Questions

  • Binary Search
  • Search Insert Position
  • Peak Element
  • Rotated Sorted Array
  • Lower Bound and Upper Bound

Final Takeaway

The First Bad Version problem is one of the most important beginner binary search problems.

It teaches:

  • boundary searching
  • divide and conquer
  • optimized API usage
  • efficient search space reduction

Understanding this problem builds a strong foundation for:

  • advanced binary search problems
  • optimization techniques
  • interview-level algorithms.