Range sum query (Fenwick tree)

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Task

📝 **Task:** Implement `BIT` with `update(i, delta)` and `range_sum(l, r)` (1-indexed).

📋 Implement the function above. Test cases run automatically.

💡 **Hint:** Re-read the theory if you get stuck — the pattern is right there.

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🎯quiz

Behavioral: "How do you push back on an unrealistic deadline?"

🎯writeFREE

Two pointers: pair sum

🎯write

Two pointers: palindrome check

🤖predictFREE

Prerequisites — what you should know before this track

⚡predictFREE

Prerequisites — what you should know before this track

✏️ Write your code here

🐍

Loading Python...

First visit only — ~5-10s. Stays cached afterward.

📊 Result

Press Run to see result...

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💬 Discussion