LeetCode 39: Combination Sum | C# Solution | Backtracking Algorithm |

🔍 In this video tutorial, I solve LeetCode problem #39: Combination Sum using #backtracking in C#. This classic problem asks us to find all unique combinations of candidates that sum to a target value, with candidates usable multiple times. #leetcode #algorithms #csharp #backtracking

⏱️ Time Complexity: O(2^target) - In the worst case, each number can be either included or excluded
🔄 Space Complexity: O(target) - For recursion depth and storing combinations

### 📋 Timestamps
00:00 - Understanding problem statement
00:47 - Using Backtracking algorithm
03:11 - Big O Notatio explained
03:45 - Optimization using Sorting
04:25 - C# Solution walk-through
05:47 - Solution analysis - runtime + memory

### 🔑 Key Concepts Covered
• Backtracking fundamentals
• Recursive decision tree
• Candidate selection & rejection
• Path tracking in recursion
• Handling duplicate combinations

### 📚 Learning Points
• How to identify backtracking problems
• Avoiding duplicate combinations
• Choosing the right base case for recursion
• Using "start" index to avoid repeating elements
• Efficiently building and tracking combinations during recursion

### 🔗 Related LeetCode Problems
• LeetCode 40: Combination Sum II (similar but with unique numbers)
• LeetCode 46: Permutations (backtracking with different constraints)
• LeetCode 78: Subsets (using similar decision tree approach)
• LeetCode 131: Palindrome Partitioning (backtracking with different condition)

### 👨‍💼 Target Audience
• Intermediate programmers preparing for coding interviews
• C# developers wanting to improve algorithm skills
• Computer Science students learning backtracking
• Anyone looking to improve their problem-solving approach

### 📝 Prerequisites
• Basic understanding of recursion
• Familiarity with C# lists and arrays
• Knowledge of time/space complexity analysis

### 🔗 Useful Links

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[code-scribbler]

💻 Full Solution Code Available!
You can find the complete C# implementation for this problem on GitHub:

🔍 Got Questions?
If any part of the solution needs clarification or if you have alternative approaches, drop a comment below! I’ll be happy to discuss.

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P.S. Don’t forget to check the video description for additional resources and related problems!
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